This study was carried out to determine the effect of dietary seaweed (Ulva lactuca) supplementation on growth performance of sheep, in vitro gas production, estimated energy, and microbial protein synthesis. A total of 18 Naimey male sheep with average live weight of 22.78 ± 0.24 kg were randomly allocated to 3 groups. Sheep in group 1 were fed a diet containing commercial feed without seaweed as a control diet, sheep in group 2 were fed the control diet with 3% seaweed, and sheep in group 3 were fed the control diet with 5% seaweed. The supplementation of seaweed had no effect (P > 0.05) on the average daily gain, feed intake, and feed conversion efficiency. The extent of gas production at 72 h was similar among the 3 groups (P > 0.05). There were no significant differences (P > 0.05) in the potential degradability, gas production rate, metabolizable energy, net energy, organic matter digestibility, and microbial protein synthesis among the experimental diets. This study concluded that dietary supplementation of seaweed (Ulva lactuca) has no effect on growth of sheep, in vitro gas production, potential degradability, estimated energy, organic matter digestibility, and microbial protein synthesis.