The mechanism maintaining widespread vegetation over the Sahara, 6000 years before present (6kBP), is investigated with an atmospheric general circulation model (GCM) focusing on an impact of convection originating above the planetary boundary layer (PBL). The multiple cloud base Arakawa-Schubert scheme (MCB) is implemented in the GCM, and it is shown that MCB significantly strengthens the impact of vegetation over the Sahara on precipitation in this region. If it is assumed that the whole Sahara was covered by vegetation at 6kBP, the total precipitation is sufficient to maintain vegetation over the south and northwest Sahara. High-valued moist static energy air ranges from surface to the middle troposphere over the northern half of Intertropical Convergence Zone (ITCZ) in summer because of strong northward moisture transport in the lower troposphere. Convection originating above the PBL prevails in this region especially when the PBL top is lowered between night and early morning and strengthens the intrusion of the ITCZ into the Sahara. Then moisture is further transported northward by the African wave disturbances enhancing precipitation over the northwestern Sahara.