The effect of the medium of C/N ratio(1∶1,2∶1,3∶1,4∶1 and 5∶1,respectively)on the growth and total lipid content of Chaetoceros muelleri was evaluated in this study.The results showed that this algae had the highest growth rate of 1.98%/d at the C/N ratio of 2∶1 and lowest growth rate of 1.69%/d at ratio of 3∶1.The total lipid content of this algae was the highest(31.26%)at the C/N ratio of 5∶1,but lowest(28.39%)at 2∶1,and the daily lipid had the highest increment,which was 0.84 ag.The daily increment of lipid content was 0.64 ag at the C/N ratio of 3∶1.The unsaturated fatty acids were also different in accumulation during the cultivation with 5 different C/N ratio mediums in this finding.The total unsaturated fatty acids content was the highest when the C/N ratio was 5∶1.In conclusion,the growth rate and total lipid accumulation of C. muelleri was best at the C/N ratio of 2∶1 in this study.