A theoretical method is required for prediction of mean field strength in land mobile radio systems, instead of the conventional empirical methods. Feasibility study of theoretical prediction using the ray-tracing method, was made in a 1.2GHz band for a model of a small-cell system. Theoretical values showed better agreement with the measured, when diffraction around the side edges of a building is taken into account. Comparison between mean field strengths in summer and winter suggested the seasonal variations in attenuation due to trees.