how to measure the number of clock cycles from execution time of a code

I am using micros() function to get the execution time of my code on Mega 2560. But can anyone tell me how to get the number of clock cycles required for executing it.

You have maybe posted this question in the wrong forum section? What if you setup several global unsigned longs (maybe t0 to t9) and then call tx = micros() function tens times in a row (not using loop) and then compare the average difference between each call. This will give you something like the execution time though interrupts will effect the results. Run it enough times and you should get an idea of the time it will take. The long winded way would be to disassemble the code and work out how many machine code instructions are needed but you will still have interrupt overheads to take into account.

Arduino.h has these 3 macro's to help you

#define clockCyclesPerMicrosecond() ( F_CPU / 1000000L )
#define clockCyclesToMicroseconds(a) ( (a) / clockCyclesPerMicrosecond() )
#define microsecondsToClockCycles(a) ( (a) * clockCyclesPerMicrosecond() )