The fat-tailed Akkaraman breed, traditionally raised in Turkey, is a breed with the best adaptation to poor feeding and harsh conditions. German Blackheaded Mutton breed originated from British downs breeds, one of breeds raised in Germany, has long mating season (about approximately two lambs a year) and a perfect adaptation to rainy country. The objective of present paper was to determine the most appropriate nonlinear model for describing complete lactation of Akkaraman and German Blackheaded Mutton x Akkaraman B-1 Crossbreed Sheep. The data were consisted of milk production records collected from 25 Akkaraman (Genotype I) and 23 German Blackheaded Mutton x Akkaraman B-1 Crossbreed Sheep (Genotype II) with 2 year of age. Daily milk yields of all ewes were recorded fortnightly from 15(th) days to 150(th) days of lactation. Quadratic, Cubic model, and Wood models were fitted to daily milk yield - time data in order to explain daily milk yield-time relationship of these ewes. These models were fitted to averages of daily milk yield of all ewes at each period for two genotypes.