Seed germination is usually enhanced by environmental cues of higher red:far-red ratio or temperature fluctuation. Seed germination shows less dependent on light and a positive relationship with temperature fluctuation with increasing of seed sizes, which ensures germination in optimal soil or litter depths in gaps.However, little support for this hypothesis has been found in forest communities.In this study, we examined seed germination of 20 temperature hardwood species from northern Japan and explored the relative importance of the red: far-red ratio(R:FR ratio) and temperature fluctuation for seed germination that differ in seed sizes. We conducted four R:FR ratios(0.0, 0.1, 0.4, 1.0) and two temperature fluctuations (20℃ and 0℃) by using temperature- and light-controlled incubators. In addition, we also conducted field experiment, in which seeds were buried in soil with different depth for 9 species with different sizes. R:FR ratio and temperature fluctuation diminished with decreasing the soil depth, while R:FR ratio showed a rapid drop compared with temperature fluctuation. R:FR ratio was more important trigger of seed germination for small-seeded species, whereas seed germination was facilitated by high diel temperature compared with high R:FR ratio for large-seeded species.