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¹ Ondokuz Mayıs Üniversitesi Veteriner Fakültesi Zootekni Anabilim Dalı, Kurupelit SAMSUN - This study was realized to determine the effects of calving season, calving year, parity and dry period on milk yield. A total of 639 lactation data from Jersey cows raised at Karaköy State Farm between 1995-2005 were used in this study. The mean values of lactation yield, lactation period and 305 days milk yield were found as 3753.93 kg; 290.16 days and 3786.98 kg, respectively. It was determined that the effect of calving year and lactation number on lactation yield and 305 days milk yield were important while the effect of dry period was important on only 305 days milk yield (P<0.01). When the 305 days milk yields were compared according to the previous dry period groups; the highest milk yield within five groups was 66-70 days of dry period and the lowest was 71-95 days of dry period. As a result, it was concluded that the optimum dry period length was between 66-70 days not to cause loss of milk yield at subsequent lactation and it was also determined that 305 days milk yield of cows at lactation 5+ was the highest. Keywords : Jersey, dry period, milk yield, environmental factors