The aim of this study was to compare the nutritive value, fermentation characteristics and in situ ruminal degradation kinetics in green forage and silage of three varieties sorghum, including Sweet, Pegah and Speedfeed by corn green forage and silage. As well as, investigation the replacing corn silage with Sweet sorghum silage on cows performance. For in vitro experiment, forages were planted in same conditions and harvested in soft-dough stage and ensilaged in four replicate in mini silos. In this study, buffering capacity of Sweet green forage was lower significantly ( p 0.05) than corn and Speedfeed green forages. Neutral detergent fiber content was significantly ( p 0.05) lower in green corn forage as compared with the Pegah and Speedfeed and also acid detergent fiber and acid detergent lignin content were higher ( p 0.05) and water soluble carbohydrate was lower significantly ( p 0.05) in Speedfeed green forage as compared with other varieties. dry matter content was significantly ( p 0.05) lower in corn silage than other silages. Neutral detergent fiber and acid detergent lignin were lower significantly ( p 0.05) in corn silage than Sweet and Speedfeed sorghum silages. Also the corn silage had a significantly ( p 0.05) lower level of residual water soluble carbohydrate than other silages. Lactate concentration was higher significantly ( p 0.05) in corn and Pegah sorghum silages than Sweet and Speedfeed (3.61, 2.19, 3.43 and 2.02 percent for corn, Sweet, Pegah and Speedfeed), and acetate percent was higher significantly ( p 0.05) in corn and Sweet sorghum silage. For in situ evaluation duplicate samples of 8 treatment (4 green forage and 4 silage) placed in nylon bags and were incubated in two rumen cannulated cow for 0, 1, 3, 6, 9, 12, 24, 48 and 72 h. Results indicated that corn and Pegah green forage had greater significantly ( p 0.05) fractional degradation rate than other two green forage and this make were significantly ( p 0.05) higher effective degradability with a passage rate 0.08/h in this two forages, but only Speedfeed green forage had lower significantly ( p 0.05) potential degradability than others. Slowly degraded fraction was more in corn silage than sorghum silages and this made a greater potential degradability (%85.25) than others, because of lower acid detergent lignin in corn silage. In in vivo study, eight multiparous Holstein cows (35.5 kg/d milk on average) were used in an repeated 4×4 latin square. Sweet sorghum was substituted for 0, 33.3, 66.6 and 100% of dry matter provided by corn silage. Intake of dry matter and other nutrient were similar among treatments but digestibility of all treatment were reduced significantly ( p 0.05) when Sweet sorghum replaced corn silage. However milk yield and composition were not different among treatments and not affected by digestibility.