Abstract: With different Tolypothrix density and 4-ethylphenol concentration,four groups of tests were conducted to study the biodegradability of 4-ethylphenol and its influencing factors.Experimental results showed that Tolypothrix was able to degrade 4-ethylphenol,the degradation process was in accordance with the second-order kinetic equation,and the degradation rate depended on Tolypothrix density and initial 4-ethylphenol concentration.With low organic compound concentration,the kinetic constant K is mainly determined by the rate of Tolypothrix growth r.