thank you stress_junkie for your response. i am going to use intel heat sink, and it comes with a thermal paste layer, so i am not going to use any other grease.
the way i was confused because of the instruction given in gigabyte mainboard manual. it says, "after placing the processor, please replace the plastic cover and lock the lever." in this way, the cpu cannot get in touch with the heat sink. moreover, in excess heat, the processor may damage the plastic which is ultimately harmful to the machine.