The treatment parameters in municipal treatment plant include COD, BOD5, SS, N and P etc. The paradoxical relationships are inevitably produced because of the different requirements on denitrification and phosphor removal. In this paper, the fundamental mechanism of biological denitrification and phosphor removal and their present situation and progress are presented. On the basis of analysis on the paradoxical relationship between biological denitrification and phosphor removal, the development of denitrification and phosphor removal process was discussed and the further study in this field was proposed.