The experiment was conducted with young trees of Satsuma Orange (Citrus unshiu MARC.) under artificially controlled climates (Phytotron). Though the average day temperature common to all 3 treatments was 18°C, the amplitudes of diurnal fluctuations in temperature were 0°C (daytime 18°C-nighttime 18°C), 10°C (23-13) and 20°C (28-8) respectively (Table 1).1. The difference of the fruit enlargement between 0°C (18-18) and 10°C (23-13) treatments was not recognized. On the contrary, under 20°C (28-8) the depression in fruit enlargement was found significantly in the former half of Stage 3 of fruit growth, but in the latter half of Stage 3 the fruit continued to grow without color development and developed the greatest fruit diameter at the end of the experiment. Greater amplitudes of diurnal fluctuation in temperature did not accelerate the fruit growth rate at all time of Stage 3 (Fig. 1, Table 2).2. Color development in the fruit rind was influenced by diurnal fluctuation in temperature. The earliest and brightest color development was observed under 0°C (18-18) and then 10°C (23-13). The palest color was found under 20°C (28-8) Color development in the fruit rind was affected not only by low night temperature, but also by high daytime temperature (Fig. 2, Tables 3, 4). The temperature 18°C at the daytime and at night seemed to be favorable for the decomposition of chlorophylls and production of carotenoids. Greater diurnal fluctuation in temperature was not necessary for the promotion of rind color development.3. Free acid concentration in the juice was influenced by diurnal fluctuation in temperature. Less diurnal fluctuation tended to produced lower acid fruit. However, acid concentration was influenced strongly by the night temperature rather than by diurnal fluctuation, that is, higher night temperature produced lower acid fruit (Fig. 4).High sugar concentration in the juice resulted from 0°C (18-18) and 10°C (23-13) treatments, but under 20°C (28-8) it was very low. Greater diurnal fluctuation in temperature did not promote the accumulation of sugar in the fruit (Fig. 4).The total soluble solids-acid ratio was the highest under 0°C (18-18). Less diurnal fluctuation in temperature produced earlier and better marketing quality (Fig. 5). Diurnal fluctuation in air temperature was not necessary for the production of better quality fruit of Satsuma orange.