As known, the constant injection mass is a criterion for measuring the thermal efficiency of diesel engines. In this study, the effects of nozzle hole diameter on diesel free-spray characteristics were investigated in constant injection mass condition. The experiment was performed in a constant volume combustion chamber equipped with a high pressure common-rail injector that can change nozzles. Three single-hole axis nozzles with different hole diameters were used. High speed camera and Schlieren visualization set-up were used to capture the spray behaviors of liquid phase and vapor phase respectively. For liquid phase spray, the higher nozzle hole diameter, the higher were the liquid phase spray penetration rate and the saturated liquid phase spray penetration length. The saturated liquid phase spray penetration length wound not grow but oscillate around different mean values at the steady stage. In addition, the increase of nozzle hole diameter increased the saturated liquid phase spray cone angle. For vapor phase spray, the variation laws of vapor phase spray penetration lengths could be divided into nonlinear increase period and linear increase period. The effects of nozzle hole diameter on vapor phase spray penetration rate were negligible in the nonlinear increase period, but the higher nozzle hole diameter, the higher was the vapor phase spray penetration rate in the linear increase period. The higher nozzle hole diameter, the higher were the increase-rate of vapor phase spray cone angles and the saturated vapor phase spray cone angles. The reduction of nozzle hole diameter increased the pure vapor phase spray penetration tip length.