To determine the thermal limit for normal spawning of Japanese whiting and to evaluate thermal effects on spawning in the normal temperature range, two experiments were conducted. In Experiment 1, the temperature of three experimental groups (Groups A, B and C) was increased from 25 to 28, 31 or 33°C, respectively. Subsequently, temperatures in all groups were decreased to 28°C. During the experimental period, the number of eggs, egg size, and hatching rate were monitored. According to the results, the upper thermal limit of normal spawning was considered to be 28–29°C, because the number of eggs and the hatching rate decreased at temperatures over 28°C. However, egg size tended to be reduced at 28°C, and was found to be strongly dependent on temperature. In Experiment 2, the number of eggs, egg size and time of spawning were compared between fish held at a constant temperature of 22°C (Group CT) and those held at increasing temperatures from 22 to 28°C (Group IT). Group IT spawned more in number and had smaller-sized eggs than did Group CT, while spawning was delayed in group IT. To identify the thermal effects on spawning of Japanese whiting, the mechanisms governing egg size must be determined.