Cercidiphyllum japonicum Sieb. et Zucc., one of the tertiary relict plant species, is the only species in a single genus, Cercidiphyllum, of the family Cercidiphyllaceae which is native to China and Japan. It was listed as one of national protected plants at the second category in China because of its extremely low ability of regeneration in natural population. In present research, the biological characteristics of seed germination were surveyed experimentally for the species, so that what conditions seed germination requires in natural regeneration would be clarified. C. japonicum during the period of fructification releases many compressed, winged seeds which are quite light with the 1000-seed weighting 0.5821±00166g. The percentage of viable seeds was tested by TTC(1.0%) to be 19.8%±1.3%, nearly close to germination percentage(GP). The imbibitional rate of seeds was 482.13%±8.54% in 14 hours. All seeds could be classified into six kinds of forms according to their surface morphology, each form having its own unique GP of seeds which differs significantly from each other, with the plump seeds of snuff color and moderate size being the highest of 91.25%±2.36%. The effect of several important environment factors on seed germination was tested for the plump seeds. The results showed that germination may be facilitated by some substance on seed capsule and the filter paper was approved to be an appropriate germination texture in comparison with sand. The germination was also affected by moisture content(MC) remarkably and the optimal range was 20%-25%. The germination could occur in an abroad range of light intensity and temperature. With exception of the decrease of GP at 5℃, temperature had no significant effect on GP, and the germination rate did, however, increase gradually with raise of temperature. Seedlings hadn′t adapted well to extremely low light intensity. To sum the above results up, the immediate causes that made it difficult for the natural populations of C. japonicum to regenerate may be inferred as follows: (1) small seeds could impose restriction for its success in competition with other plant species; (2)Quantity and quality of seeds were both decreased by erosion of habitat fragmentation; (3) seed germination in low temperature conditions led population to be exposed to danger, although favorable to accumulation of metabolic substances; (4) growth of root and stem was negatively influenced by extremely low light intensity under forestry; (5) Rapid germination could be possibly initiated by positive correlation with MC.