A 12‐week feeding trial was conducted to evaluate the effects of dietary phosphorus on growth performance, whole‐body composition, liver phosphorus metabolism enzyme and digestive enzyme activities of coho salmon (Oncorhynchus kisutch) alevins. Six experimental diets were formulated with graded phosphorus levels of 0.3%, 0.59%, 0.73%, 0.85%, 1.05% and 1.19% respectively. Each diet was fed to triplicate groups of fish (0.36 ± 0.01 g), which were randomly assigned to 18 tanks (water volume 240‐L), with each stocked initially with 50 fish. The specific growth rate (SGR), whole‐body and vertebrae phosphorus contents, lipase and amylase were significantly improved with dietary phosphorus level increased from 0.3% to 0.85% (p < .05). The feed conversion rate, alkaline phosphatase and creatine kinase activities were significantly decreased with dietary phosphorus level increased from 0.3% to 0.73% (p < .05). The hepatosomatic index, intestosomatic index, condition factor and whole‐body moisture, crude protein, crude lipid and ash were not affected by dietary treatments (p > .05). More accurate information on dietary phosphorus requirement was obtained by cubic curve regression analysis of SGR, whole‐body and vertebrae phosphorus contents, which indicated that phosphorus dietary requirement levels for coho salmon alevins were 0.95%, 1.06% and 1.11% respectively.