This paper explores the consequences of introducing a monopolistic competition in an intertemporal two-sector small open economy model which produces traded and non traded goods. It is assumed that the non traded sector is the locus of the imperfectly competition. Our analysis shows that markup depends on the composition of aggregate non traded demand and is therefore endogenously determined in the model. Calibrating the model with OECD parameters, the effects of fiscal and technological shocks are simulated. Our findings are as follows. First, the model is consistent with the observed saving-investment correlations found in the data. Second, unlike the perfectly framework and in accordance with empirical studies, fiscal shocks cause real appreciation of the relative price of non traded goods, which in turn enlarges the responses of current account and investment. Third, the model is consistent with the empirical report that technological shocks result in current account deficits and investment rises. Fourth, the strength of the relative price appreciation following sector productivity differentials, i.e. the Balassa-Samuelson effect, is affected by the monopolistic competition hypothesis. Assume perfect competition when it is not, biases upward estimates of the Balassa-Samuelson effect.