A numerical simulation is carried out mixed convection in horizontal channel with a heat source from below of rectangular cavity. Finite volume method was employed for the discretization and PISO algorithm was used for calculating pressure term. The parameters governing the problem are the Reynolds number ($10^{-2}{leq}Re{leq}50$), the Rayleigh number ($10^3{leq}Ra{leq}2.06{ imes}10^5$), the Prandtl number ($0.72{leq}Pr{leq}909$), the aspect ratio ($0.5{leq}AR=W/H{leq}2$) and the angle of inclination ($0^{circ}{ heta}60^{circ}$). Mean Nusselt number distributions were obtained and effect of Reynolds number, Rayleigh number and Prandtl number on mixed convection in the horizontal channel with rectangular cavity were investigated.