Abstract: The growth character of Karenia mikimotoi was studied in culture medium at different ambient ratios of N/P(4:1,8:1,16:1,32:1,80:1).The results showed that N/P ratio significantly affected the growth of Karenia mikimotoi.Too high or too low N/P ratio was not suitable for Karenia mikimotoi growth.The algae grew fastest under 32 N/P ratios,and the maximum specific growth rate was 0.33 d^-1. Karenia mikimotoi demanded more nitrogen than phosphorus for growth,and it could maintain a longer growth cycle in the appropriate phosphorus limited environment.The main reason of Karenia mikimotoi red tides was not the low N/P ratio in the environment,but was a large number of nutrient consumption,especially nitrogen.