can constants defined in a file be used in other file without defining ?

Create your own header file, for example "keys.h"
and include it in any file which use
defines of the header file.

/* header file */
#if !defined(KEYS_H_HDR)
#define KEYS_H_HDR

#define key0 0x35
...

#endif

/* in source file use (for example) ... */
#include "keys.h"

Vaclav

"Does keil have any provision..."

this is absolutely standard 'C' stuff - nothing special about Keil!

I suggest that you need to get a good, standard 'C' text book.

Thanks Vaclav & Andrew
I am new to programming in C,book to which i had reffered did not carry detail about
this.

Is it neccessary that header file created
by us should be kept in "inc" folder.

doubt!! --> KEYS_H_HDR
(#ifndef KEYS_H_HDR
#define KEYS_H_HDR )
Does this should be same as header file name ?

"book to which i had reffered did not carry detail about this."

In that case, it does not qualify as a "good" book on 'C' - this is fundamental stuff.

"Is it neccessary that header file created by us should be kept in "inc" folder."

No.
As with any application trying to find a file, there are certain default locations where the compiler will look, and a certain order in which it will try the alternatives - or you can explicitly state the file's location by includinging a full or relative path.
You need to read the Compiler Manual for details. For C51, see the INCDIR directive.

Arjun asked:
> doubt!! --> KEYS_H_HDR
> (#ifndef KEYS_H_HDR
> #define KEYS_H_HDR )
> Does this should be same as header file > name ?

The label name is arbitrary, but it keeps things neat to make it the same as the header filename.
This is a very common technique to avoid "multiple definition" errors if the same header file is referred to twice. (The first reference will "define" KEYS_H_HDR, so a second reference will "comment out" the entire contents of the header file.

If your C textbook does not refer to header files, then get another one. This is a fundamental part of the C language.