This paper demonstrates the relation between the heat island intensity and city size indices/urban canopy characteristics. This is accomplished by analyzing the data at 6 settlements on the Nagano basin, Japan, whose population ranges from 3, 300 to 360, 000. The results show that heat island intensities on fine nights and cloudy nights, ΔT_u-r are related to the logarithm of the settlement population P, which has one of the highest correlations to the heat island intensity in a city size index such as city area, DID (Densely Inhabitant District) area, DID population and so on. The gradient of regression between ΔT_u-r and P is larger on fine nights than on cloudy nights, and is larger on nights with weak wind than on nights with strong wind. The correlation of population with maximum heat island intensity is remarkably high for Japanese settlements.