Disintegration processes of a liquid jet injected from a coaxial dual nozzle into a stationary atmosphere were investigated experimentally. As the primary study in a series to clarify the effect of internal disturbance in a liquid column on the disintegration processes, velocity profiles which are deformed from the Poiseuille flow were produced by using a coaxial dual nozzle. The test fluid employed was tap water and the injection was vertically downward. The breakup length was measured by a stroboscope and a camera under different conditions of outlet velocity profile. The influence of the diameter ratio of the dual nozzle on the breakup length was also investigated. The experimental results indicate that the breakup length is strongly affected by the internal velocity difference rather than the rim disturbance of the inner nozzle. This suggests that the internal shear flow or disturbance in the liquid column caused by the internal velocity difference promotes the disintegration of the liquid jet.