In order to study the effect of the over-temperature on the microstructures and high cycle fatigue properties of DD6 single crystal superalloy, the alloy after standard heat treatment was hold at 1200℃ and 1200℃/50MPa for 1h respectively, which simulated different over-temperature process, and was then air cooled. The microstructures and high cycle fatigue properties of DD6 alloy with standard heat treatment, 1200℃/1h, 1200℃/50MPa/1h at 800℃ were investigated by SEM and rotary benging fatigue testing machine. The results show that after heat-treatment at 1200℃ for 1h, the size of γ′ particles becomes larger slightly and quite nonuniform, serrated γ/γ′ interface appeares and a small number of fine secondary γ′-particles precipitate in the γ matrix channels. After heat-treatment at 1200℃/50MPa for 1h, the size of γ′ particles also becomes larger while also quite nonuniform; a large number of fine secondary γ′ particles precipitate in the γ matrix channels which have become wider significantly and the directional coarsening of γ′ particles occurs. In addition, the rafting of the sheet-like morphology of γ′ particles is formed along the [100] and [010] directions after fatigue test. The high cycle fatigue properties of DD6 alloy by standard heat treatment are higher than that of the alloy by 1200℃/1h, but are similar to the alloy by 1200℃/50MPa/1h. Analysis on fracture surfaces of three heat treatment process at 800℃ demonstrates that quasi-cleavage mode is observed.