Using a four‐moment method of the kinetic theory of gases, an analytical expression is derived for the problem of vapor condensation on and evaporation from spherical liquid droplets when the droplet is not in equilibrium with its surrounding vapor in temperature and pressure. Unlike the linearized theories, the result for the mass flux to or from the droplet surface is obtained for a general case of finite temperature and pressure differences between the droplet and its vapor. It is believed, therefore, that the present nonlinear theory should be more realistic in predicting the rate of vapor condensation or evaporation phenomena of a droplet.