In this experiment, effects of soil moisture content, cultural temperature and a seed size on the strength of plulnule-elongation in paddy rice plants were examined using unbonded gauge type transducer (UT 1kg) and balancing null recorder in darkness. This strength shows mechanical vigour of plumule to push a thing up with the tip of it. The plumules of rice plants were raised under the covering soil of l.6-6.5 cm.The following results were obtained. 1. Effect of soil moisture content on the strength of plumule-elongation in a paddy rice variety, Hoyoku, in japonica type was little under the conditions ranging from l0 to 15 percent of moisture content. The strength decreased with increasing of soil moisture content. It was more evident in short plumules than in long plumules (Table l, Fig. 2). 2. The strength of pltlmule-elongation in paddy rice plants which did not elongate mesocotyl (Hoyoku in japonica type) was maximum in the plumules of 1cm long, and it decreased with growing of plumules. In particular, the strength decreased strikingly in the plumules of about 4 cm long (Fig. 3. 4). On the other hand, the strength in rice plants which elongated mesocotyl (treated-Hoyoku, Hatishail in indica type) was almost the same strength in all stages of plumules (1∼6cm) (Fig. 4). 3. The strength of plumule-elongation in a paddy rice plant, Hoyoku, was more vigorus in the plumules which were cultured under warm conditions (20-30°C) than those under cool condition (17°C). At 17°C, the strength was about 1/4 as Compared with that of 30°C (Table 3). 4. In some paddy rice varieties in japonica type, possitive correlation was observed between the strength of plumule-elongation and a seed size (r=0.512) (Fig. 5).Also, in a paddy rice variety, Hoyoku, the more seed size was large, the more strength of plumule-elongation was vigorous (Table 4).