An external hazard curve of a forest fire is evaluated based on a logic tree. The logic tree consists domains of "forest fire breakout and spread conditions ", "weather condition", and "vegetation and topographical conditions". A location nearby a typical nuclear power plant site in Japan was selected for our studies. The frequency of a large forest fire of the location is approximately 1/5 of the average in Japan. Forest fire breakout points were selected through deterministic considerations on typical forest fire causes in Japan. The weather conditions around the location are represented by two parameter sets of "temperature-humidity " and "wind direction-wind speed". An appearance frequency database was prepared for weather parameters (i.e. wind speed and humidity) sensitive to forest fire intensities, whereas a nonsensitive branch (i.e. temperature) was eliminated from the logic tree. A number of forest fire simulations were performed to obtain a response surface for a frontal fireline intensity at different combinations of wind speed and humidity. The hazard curve is therefore evaluated by a Monte Carlo simulation where one sample gives a unique intensity from the response surface and its frequency is given by the combination of the branching probabilities in the logic tree. The evaluated hazard curve is such that the annual exceedance probability is about 1.0×10-4per year for the frontal fireline intensity of 200 kW/m and about 1.3×10-5per year for 300 kW/m.