Younger generations encounter much more challenges than their (grand)parents in fighting climate change and experience more radical behavioral changes in achieving net zero carbon target by 2050. However, there is insufficient research on energy-saving behavioral mechanisms of young people and the gender effect on their perceptions and practices. This study aims to explore gender differences in reasoning energy-saving behaviors of university students. First, a social psychological model was proposed by expanding the theory of planned behavior to expound energy-saving behaviors. Then, the proposed model was tested using structural equation modelling methods with data obtained from 1021 university students in Hong Kong. Next, gender differences in energy-saving behaviors were examined with multigroup analysis and comparative statistical methods. Results show that gender differences existed evidently in energy-saving perceptions and practices of university students in favor of females. Behavioral intentions of males and females were predicted by different combinations of beliefs, though they shared similar “belief - intention & control - behavior” logics. Gender effect sizes ranged from trivial to moderate and overestimation or neglect of meaningful gender differences should be avoided. The findings enable a better understanding of gender effect in pro-environmental domain and facilitate the transformation to low carbon lifestyles.