Ammonia synthesis directly from nitrogen and hydrogen was attempted by use of microwave plasma. First, the ammonia yield was investigated using active alumina as a catalyst in a microwave plasma. Next, the influence of the number of microwave radiation ports on ammonia synthesis was investigated. It was found that activated alumina has the effect of improving the ammonia yield. It was also found that the efficiency of synthesis can be raised by decreasing the reaction pressure, by decreasing the microwave output power and by generating a wider range of microwave plasma.