Can I use atmega url in board manager for at89c51 to install board support in Arduino software.......I m using at89c51 as a alternative for Arduino......help me with a library or board manager to support at89c51 in Arduino software.....

First thing I find when googling for arduino at89c51 is http://forum.arduino.cc/index.php?topic=172862.0

Arduino does not support 8051 based chipsets.

The now free MCU 8051 IDE supports C with the SDCC compiler. Atmel/Microchip has an antiquated IDE that runs under windows that supports the device family but I seem to remember it as ASM only, no C. If interested, I would ask over at AVRFreaks for a link, it’s still in the Atmel archives AFAIK.