i found the answer at keil's site.
http://www.keil.com/support/docs/2069.htm
In some cases, uVision interrogates the MON51 target using XON and XOFF characters. If you receive 0x11 or 0x13 characters, this may be the problem you are experiencing. To correct this problem, you must ignore XON and XOFF in your program. If you use the standard GETKEY routine, the following substitute function may be added to your project:
#include <reg51.h>

char _getkey () {
char c;

do
{
while (!RI);
c = SBUF;
RI = 0;
}
while (c == 0x11 || c == 0x13); // IGNORE XON and XOFF

return (c);
}

it works now!!