Abstract: The Fukushima Daiichi nuclear power plant accident on March 11, 2011 caused the month-long discharge of radioactive materials into the atmosphere. By using numerical prediction model WRFV2.2.1 and dispersion model CALPUFF, atmospheric transport and dispersion of radioactive materials was simulated during the accident. The method of a reverse estimation of source term into environment was applied by combining environmental monitoring data with atmospheric dispersion simulations under the assumption of unit release rate. Preliminary estimation of release amounts of ¹³¹I and ¹³⁷Cs accidentally discharged from Unit 1 through Unit 3 of the Fukushima Daiichi nuclear power plant into the atmosphere was obtained, based on the environmental monitoring data. Release amounts of ¹³¹I and ¹³⁷Cs are 8.6×10¹⁶Bq and 8.6×10¹⁵Bq, respectively.