Using Arduino Uno as ISP to program Mega 2560
Find the file called "boards.txt" located in your Arduino installation directory. In my case, the path is "arduino-1.6.5/hardware/arduino/avr/boards.txt". Open it with your favourite text editor and locate the ATMEGA2560 section. (not in notepad, it breaks the file format, notepad++ my choice)

in boards.txt


back-up the file incase you break it,
find the line :
mega.menu.cpu.atmega2560.bootloader.high_fuses=0xD8
and change it with :
mega.menu.cpu.atmega2560.bootloader.high_fuses=0xD9


Source : http://soc.ninja/documentation/programming-atmega2560-with-arduino-uno-or-mega/#rating-74