The accurate assessment of pedestrian-level wind environment is one of the major issues in the development of urban areas, which is usually evaluated on the basis of wind tunnel experiments. The accuracy of the assessment is directly related to that of wind speed ratio obtained from the wind tunnel experiments. There are some factors producing a difference between the predicted values from the wind tunnel experiments and actual values from field measurements. The present paper focuses on the following two factors. One is the effect of deciduous trees that are not reproduced in the wind tunnel models in most cases. However, there are many deciduous trees in urban areas, which may reduce the wind speeds near the ground significantly. The other is the effect of the wind direction fluctuation at the reference point. The range of fluctuation is generally much smaller in the wind tunnel flow than in the natural winds. The effects of deciduous trees and wind direction fluctuation on the wind speed ratio are investigated based on the results of wind tunnel experiments and field measurements at various locations in Tokyo.