It has been established that mouse oocytes grow under appropriate culture conditions. To be able to utilize a broad range of growing oocytes, a way of estimating the optimal culture period is required. In the present study, a detailed analysis was conducted of the relationship between the initial oocyte size and necessary length of culture period to achieve full growth. Oocyte-granulosa cell complexes were obtained from juvenile mice, and classified into three groups according to oocyte size: 50.0–54.5, 55.0–9.5, and 60.0–64.5 μm. The complexes were cultured for various periods up to 12 days. Mean oocyte size increased in a linear fashion, eventually reaching the full size for mouse oocytes. Meiotic competence was acquired during the last 4 days of culture, when oocytes approached full size in all size classes. During this period, the rate of oocytes incompetent to resume meiosis decreased sharply, while that of oocytes having competence to progress to metaphase II increased. This relationship was particularly clear cut in the large- and middle-size classes. Thus, appropriate length of culture period is mostly determined by the oocyte size at the start of culture. Considerable attention should be paid to the oocyte size as well as to the duration of culture period.