The effect of distance of energetic argon ion irradiation is studied on ZnSe stoichiometric film having compressive lattice strain and improve the refractive index. An increase in grain size and decrease in lattice strain was observed with decreasing irradiation distance (or increasing ion energy). The decrease in distance of energetic argon ion irradiation can effectively improve the transmittance. Both of film thickness and refractive index were calculated in terms of Swanepoel's method with high precision. The film thickness and refractive index of ZnSe thin films decrease with increasing the energetic argon ion irradiation i.e. decrease the distance between the anode and the sample.