The combination effects of light and air temperature, which were expressed as photothermal ratio (PTR), on the growth and sex expression of three monoecious cucumbers (Cucumis sativus L. cv. “Xintaimici”, “Jinyan 3” and “Jinyan 4”) were studied with hydroponics in this research. The results showed that with the increase of PTR, different growth responses were observed in the three cucumber cultivars. In contrast, high PTR (H-PTR, 0.86 mol·m^-2·degree-day^-1) significantly increased the total number of female nodes and decreased the total number of male nodes of the three monoecious cucumber cultivars compared with cucumbers grown under low PTR (L-PTR, 0.64 mol·m^-2·degree-day^-1). More photoassimilate was partitioned to fruits of three cucumber cultivars with the increase of PTR, indicating that PTR-mediated femaleness of monoecious cucumber might be attributed to the enhancement of photoassimilate translocation from source organs to sink organs.