Moist convection of Jupiter's atmosphere is examined using a large-domain two-dimensional fluid dynamical model with simplified cloud microphysics of water. The result shows that the water condensation level acts as a dynamical and compositional boundary. The convection below the condensation level is characterized by a steady regular cellular structure and a homogeneous distribution of water mixing ratio. Above the condensation level, cloud elements accompanied by the upward motion develop and disappear irregularly but successively, and water mixing ratio is highly inhomogeneous. The horizontal average of mixing ratio decreases rapidly with height just above the condensation level, resulting in a distinctive stable layer at 5 bar. The stable layer prevents the air masses above and below it from mixing with each other. As a result, the upper dry air does not reach 20 bar level, where the Galileo probe observed low humidity. ¿ 2000 American Geophysical Union