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.