This paper develops a dynamic general equilibrium model and studies structural change in a small open economy with two tradable sectors, agriculture and manufacturing, and a non-tradable sector, services. In addition to obtaining results for a falling employment share of agriculture and a rising share of services, we demonstrate analytically the hump-shaped share of manufacturing by identifying two countervailing effects: the productivity effect and the BalassaSamuelson effect. The first effect, arising from differential rates of productivity growth among sectors, increases the share of manufacturing; the second effect, together with low rates of substitution between products, enhances the service sector and eventually draws labour from the manufacturing sector. At the aggregate level, however, the economy maintains a constant rate of growth. We calibrate the model with data from South Korea and find that the calibration fits the country's historical path of structural change.