Generation of negative air ions (NAI) by indoor plants which belong to 25 families (Araceae, Liliaceae, etc.) was studied. Those plants which can produce high concentration of NAI to maintain, even to increase the amount of NAI indoor was screened out. And they were capable of improving the quality of indoor environment. Measurements of NAI concentration generated by 36 species of plants were taken inside an airtight glass container. The results showed facts as following: Ficus lyrata produced 76 ions/cm3 which was the highest while Crassula portulacea produced 26 ions/cm3, the lowest. By mean of concentration in a whole day, Asparagus myrioeladus and Begonia semperflorens respectively produced a highest level of 43 ions/cm3, which was also 2.4 times as controlled experiment. In the state of nature, most plants produced higher NAI concentration during daytime than during night. And the concentrations of NAI generated by most plants kept steady at different times of day while a few plants kept larger rate of change in the concentrations of NAI.