Can i use arduino code, without using arduino board but just using MCU included I am developing a product, and to ease my job i am planning to use MCU inside the arduino with the library of Arduino.

Since all the registers are already defined inside the library, and all required things to registers of corresponding MCU are done to "make a pin input/output , enable interrupt , enable UART , enable I2C". I think that i can directly use the arduino coding enviroment even if i use MCU seperately. 

Is it possible and how?  (can i use it in commertial product, i have already prototyped it with arduino)
Yes, you can. Once you have a .hex file for a specific MCU and environment you can use a programmer to upload it to a discrete MCU.