Abstract: In order to find out the physiological mechanism of hybrid rice, some phy- siological and biochemical functions of the hybrid strain, "Nan U-2" were studied comparatirely with the improved variety of ordinary late rice, "Kuang Yu-73". Preliminary results obtained are summarized as follows: 1. The photosynthetic efficiency per unit leaf area "Nan U-2" was slightly lower than that of "Kuang Yu—73”. The formers greater yield in grain output is due chiefly to its larger leaf area. 2. As compared with "Kuang Yu-73", "Nan U-2" showed lower respiration intensity while its photorespiration rate was still lower. 3. The activity of the root system of "Nan U-2" was found to be somewhat higher than that of "Kuang Yu-73". "Nan U-2" also showed a more developed and intensive root system. 4. Utilization of nutrients and rate of food material transport of "Nan U- 2" exceeded that of "Knang Yu-73". 5. On the basis mentioned above further physiological and biochemical analy- ses of "Nan U-2' as well as its three lines of parents were made. The results are summarized as follows: 1) Photosynthesis, respiration and root system activity of "Nan U-2" was influ- enced mainly from its male parent, whereas its lower photorespiration rate seems more likely to be an inheritance from its male sterile parent. 2) Photosynthetic maxim peak of "Nan U-2" at the anterier stage appeared earlier than that of its parents. The duration of high efficiency lasted longer. Respiration intensity throughout its whole life cycle, was lower than that of its parents. 3) Aeration of roots of "Nan U-2" was better than that of its parents. The activity of its root system at the early stage was lower, but it surpassed that of its parents with a sudden leap at the middle stage and maintained higher activity throughout its last stage. 4) The potassium absorbing capacity of "Nan U-2" is especially high. The requirement of potassium of "Nan U-2" greatly exceeds that of its parents.