A student

I have some skills, but none of them includes telepathy (I doubt whether many others here do). How about copy-pasting the error message(s)?

this is the code:
INCLUDE <REG52.h>
BACK:MOV A,#55H MOV P1,A ACALL DELAY MOV A,#AAH MOV P1,A ACALL DELAY SJMP BACK

DELAY: MOV R0,#11 H1:MOV R1,#248 H2:MOV R3,#255 H3:DJNZ R3,H3 DJNZ R1,H2 DJNZ R0,H1 RET END
error message: Build target 'Target 1'
assembling TEST2.A51...
TEST2.A51(1): error A9: SYNTAX ERROR
TEST2.A51(5): error A45: UNDEFINED SYMBOL (PASS-2)
Target not created

you forgot to:
1. use your previous thread
2. use the proper tags used to post source code (pre and /pre)

what kind of header should i use in a51
just like we use #inclde<> in c51?

The kind that is described in the A51 Manual!

http://www.keil.com/support/man/docs/a51/

Specifically: http://www.keil.com/support/man/docs/a51/a51_include.htm

Note that "The Ax51 Assembler includes a standard C preprocessor that is identical with the macro preprocessor in the Cx51 Compiler (with a few minor exceptions"
http://www.keil.com/support/man/docs/a51/a51_mp_c.htm

you can also reference their "TEMPLATE.A51" file:

C:\...\Keil\C51\ASM\TEMPLATE.A51

Just a re-write...

#INCLUDE <REG52.h>

BACK:
   mov   A,#(55h)
   mov   P1,A

   ACALL DELAY

   mov   A,#(AAh)
   mov   P1,A

   ACALL DELAY

   SJMP  BACK

DELAY:
   mov   R0,#(11)
H1:
   mov   R1,#(248)
H2:
   mov   R3,#(255)
H3:
   DJNZ   R3,H3
   DJNZ   R1,H2
   DJNZ   R0,H1
RET

END

My preference is to localize the .INC or .H file so it is located with the general source code directory, thus keeping the whole program intact:

#include "REG52.h"

--Cpt. Vince Foster
2nd Cannon Place
Fort Marcy Park, VA

FROM THE TEMPLATE FILE IN KEIL A51.I USED:
$NOMOD51
#include <reg52.h> ;THE ERROR MESSAGE WAS:

Build target 'Target 1'
assembling TEST2.A51...
TEST2.A51(7): error A45: UNDEFINED SYMBOL (PASS-2)
Target not created.

PLS HELP ME SOLVE THIS PROBLEM!

You are forward referencing the DELAY routine. Is that the problem (I don't want to test it myself now):

(or you might need to use the $INCLUDE method--depending upon the revision of KEIL I guess)...

#INCLUDE <REG52.h>

DELAY:
   mov   R0,#(11)
H1:
   mov   R1,#(248)
H2:
   mov   R3,#(255)
H3:
   DJNZ   R3,H3
   DJNZ   R1,H2
   DJNZ   R0,H1
RET


BACK:
   mov   A,#(55h)
   mov   P1,A

   ACALL DELAY

   mov   A,#(AAh)
   mov   P1,A

   ACALL DELAY

   SJMP  BACK

END

--Cpt. Vince Foster
2nd Cannon Place
Fort Marcy Park, VA

i try the correction u made above and this was the error message: Build target 'Target 1'
assembling TEST2.A51...
TEST2.A51(1): error A315: unknown #directive 'INCLUDE'
TEST2.A51(23): error A45: UNDEFINED SYMBOL (PASS-2)
Target not created

Try the leading zero in the mov A,#(AAh) to mov A,#(0AAh)

#INCLUDE <REG52.h>

DELAY:
   mov   R0,#(11)
H1:
   mov   R1,#(248)
H2:
   mov   R3,#(255)
H3:
   DJNZ   R3,H3
   DJNZ   R1,H2
   DJNZ   R0,H1
RET


BACK:
   mov   A,#(55h)
   mov   P1,A

   ACALL DELAY

   mov   A,#(0AAh)
   mov   P1,A

   ACALL DELAY

   SJMP  BACK

END

--Cpt. Vince Foster
2nd Cannon Place
Fort Marcy Park, VA

Captain, if you continue posting source code at this rate, Jack Sprat is likely to start complaining about your "cowboy" debugging technics...

the error is now : Build target 'Target 1'
assembling TEST2.A51...
TEST2.A51(1): error A315: unknown #directive 'INCLUDE'
Target not created

No kidding.

--Cpt. Vince Foster
2nd Cannon Place
Fort Marcy Park, VA

i was just trying to see how i can correct this problem.so donot be angry.

Don't make me come down there and fix it!
(At this point, you may need a sardine's take on it)

Are you sure about the #include <REG52.H> ?
Try localizing it (move REG52.H to the source file directory) and do the #include "REG52.H" thing.

(I really don't want to create a project just do debug this stupid, err, sample code).

--Cpt. Vince Foster
2nd Cannon Place
Fort Marcy Park, VA