The stability property of the tropical intraseasonal oscillation (ISO) during 1 November 2003 to 31 January 2004 is examined using tropical bred vectors obtained from the operational numerical weather forecast system of the Japan Meteorological Agency. The tropical bred vectors are produced by a modified operational breeding cycle in which the perturbation is damped over the extratropics and rescaled by 3.3% of the climatological variance of the 200-hPa velocity potential in the tropics. At least two growing tropical bred vectors that have similar spatial structure to the observed dry Kelvin waves are obtained: dominant zonal wave number 1 components propagating eastward with phase speed of 30 m s-1. The time-mean growth rate of the fastest growing tropical bred vector has a positive value of 0.1 day-1. Although this growth rate is smaller than that of extratropical baroclinic instability, this result suggests that the tropical ISO is unstable to infinitesimal perturbations.