In summer season, the characteristic of bull semen is generally not so good, which is usually called as summer sterility. However, this phenomenon has not been investigated in the Ryukyu Islands, which are under a subtropical oceanic climate. The purposes of this study were 1) to estimate variance components of the main and environmental factors, i.e. bull, month, ejaculation, collection interval, etc. for semen characteristics, 2) to estimate the effects of month, ejaculation and collection interval on semen characteristics using the least-squares means, 3) to investigate seasonal change in semen characteristics in the Island of Okinawa. The data were recorded from May, 1993 to Dec., 1997,collected at the Okinawa Prefectural Livestock Experimental Station. The variance components of the factors and leastsquares means of each level of the factors for semen characteristics were estimated by VARCOMP and GLM procedures of SAS. Results of this study showed that the variance components of bull indicated the highest values in all semen characteristics. Meanwhile, the variance components of month showed lower values in all semen characteristics and the percentages to total variances showed from 0.0 to 2.0 %. The first ejaculation and approximately five-day intervals showed the best ejaculation and better collection interval for semen characteristics, respectively. Concerning to the variation of seasonal factor, the least-squares means of sperm concentration and total sperm showed the lowest values in June when the highest relative humidity was observed. However, the sperm concentration and total sperm were recovered during summer season of August. The other semen characteristics did not show certainly pat-tem due to the seasonal variation. Jpn. J. Livest. Management, 35(2) : 31-40,1999 Received July 13,1998 Accepted June 15,1999