Abstract: To study the displacement and replacement effect during gas injection into coal seam for displacing meth- ane,and the leading function between two effects,a simulation experiment of gas injection into coal seam was conduc- ted under the condition of one dimensional stress and layer pre-press formed coal seam by using self-developed experi- ment device. The results indicate that the displacement effect and replacement effect is always occurring throughout the whole process of gas injection. Under their combined action, the methane is discharged. At the early stage within 14 min of gas injection,the replacement effect plays a leading role. The injected nitrogen is all detained in coal sam- ple because of the adsorption by coal or filling with free space such as coal fracture;and it shows no nitrogen discharge out in the phase,meanwhile a large amount of methane is desorbed and discharged. In the period within 14-200 min of injection experiment,the dominant position of the displacement effect will be gradually lost,and begin to enter the transition phase which the displacement effect is weakened and the replacement effect is enhanced. At the last stage af- ter 400 min,it is the period that the displacement and the replacement are combined to balance each other. But the to- tal efficiency of displacement and replacement is at a low level. According to the experimental results that the dis- charge methane efficiency is low after 400 min (6． 67 hours). Therefore,the study suggests that the period of gas in- jection should be controlled within eight hours timeframe.