To maximize net gain of a tree, leaves must be replaced when net gain of a leaf per unit time over the leaf's life span is maximum. A model in which leaf longevity is determined to maximize the net gain of a leaf per unit time is constructed. The model predicts that leaf longevity is short when initial net photosynthetic rate of the leaf is large, long when the construction cost of the leaf is large, and short when the decrease in net photosynthetic rate with time is large. The model describes leaf habit (deciduousness and evergreenness) with the length of the favorable period for photosynthesis within a year and simulates distributional pattern of leaf habit along latitudes. The percentages of evergreenness decrease with decreasing favorable-period length and reach the minimum at an intermediate length of the favorable period but increase again with a decrease in the length of the favorable period. A bimodal distributional pattern with two peaks, one at lower and the other at higher latitudes, is observed for the percentages of evergreenness. Percentages of deciduousness show a unimodal distribution pattern with a peak at midlatitude.