As known, the constant injection mass is a criterion for measuring the thermal efficiency of diesel engine. In this study, the effects of nozzle hole diameter on diesel free-spray characteristics were investigated in the condition of constant injection mass. The experiment was performed in a constant volume combustion chamber. A high pressure common-rail injector that can change nozzles was applied. Three single-hole nozzles with different hole diameter were used. High speed camera and Schlieren visualization set-up were used to capture the behaviors of liquid phase spray and vapor phase spray respectively. For liquid phase spray, the spray penetration length increased as injection time elapsed until the spray reached the saturated liquid phase spray penetration length. The reduction of hole diameter reduced the time that the liquid phase spray penetration length reached the saturated value. Higher hole diameter caused slightly higher liquid phase spray penetration rate and higher saturated liquid phase spray penetration length. At the saturated stage, the liquid phase spray penetration length won’t grow but oscillate around a mean value. After injection stop, the liquid phase spray won’t dissipate immediately but stay the saturated stage a transient period. In addition, higher hole diameter caused higher 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. In the nonlinear increase period, the effects of hole diameter on vapor phase spray penetration rate were negligible, but higher hole diameter caused higher vapor phase spray penetration rate in the linear increase period. And the differences were reduction as the injection pressures increase. Higher hole diameter caused higher increase-rate of vapor phase spray cone angles and higher saturated vapor phase spray cone angles.