The effects of trade policy are investigated for international duopoly with several markets. Deriving the relationship existing between the total outputs of the home and foreign firms, we analyze two cases, in one of which--Krugman's case--the home and foreign firms' marginal costs are both decreasing and in the other, the home firm's marginal cost is increasing but the foreign one is decreasing. Stability conditions are derived for an iterative scheme of computing the equilibrium total output and for the firm's output adjustment system with discrete time scale.