The instability of a swift and narrow current flowing eastward in the mid-latitude open ocean is studied with a multi-mode quasi-geostrophic model. Linear analysis shows that there exist two groups of unstable waves: short and long. The phase velocity of the former is much larger; the growth rate of both is almost proportional to the maximum speed of the baroclinic current. As the width of the jet increases, the growth rate of the long waves increases and that of the short waves decreases.