A method for compressing instruction is provided, which includes the following steps. Analyze a program code to be executed by a processor to find one or more instruction groups in the program code according to a preset condition. Each of the instruction groups includes one or more instructions in sequential order. Sort the one or more instruction groups according to a cost function of each of the one or more instruction groups. Put the first X of the sorted one or more instruction groups into an instruction table. X is a value determined according to the cost function. Replace each of the one or more instruction groups in the program code that are put into the instruction table with a corresponding execution-on-instruction-table (EIT) instruction. The EIT instruction has a parameter referring to the corresponding instruction group in the instruction table.