Abstract: 12 different varieties of kenaf (Hibiscus cannabinus L.) were used to study the effects of some inner and outer regulating factors on their nitrate reductase activities (NRAs).According to the results,the ingredients of extract,centrifugal speed per hour, illumination, temperature, substrates and seedling age could influence the NRAs. Furthermore, the stability of determining the NRAs of kenaf varieties in vitro, their rate of attenuation, and their relation to the variety fertilizer tolerance were also studied. The preliminary results show as follows: (1) the discrepancy in regulating functions of the NRAs between the varieties can affect the synthesis of enzyme and its stability of regulating capacity; (2)the crude enzyme preserved under the conditions of different gradients of temperature still has the functions of continuing to regulate, transcribe and synthetize the nitrate reductase. The attenuation speed of the variety nitrate reductase in vitro becomes faster as the temperature ri-ses;(3) the correlation between the NRAs and the variety fertilizer tolerance of kenaf is negative.