The environmental monitoring system for radiation exposure due to light water reactor (LWR) nuclear power plant plume is described. This system, employing a NaI(Tl) scintillation detector with an exposure rate conversion circuit, has the capability of highly accurate evaluation of radiation exposure due to an LWR plume with 0.15 million R/yr (39 nC/kg.yr) overevaluation and 0.05 million R/yr (13 nC/kg.yr) underevaluation. Plume exposure is identified by combining the time variation property of the exposure rate with meteorological parameters. Outlines of the present system are described, including monitoring results obtained over a 98-day period by tentative system at an LWR site boundary.