The study investigated the impacts of buoyancy force and heat source on a reactive third grade fluid flow within parallel channel. The results obtained for the coupled equations governing the fluid flow which are strongly nonlinear are secured using modified Adomian decomposition method (mADM). The present result is compared with the one in Makinde (2009) where the influence of buoyancy force and heat source were not accounted for. The significant effects of buoyancy force and heat source are noticed to increase the fluid motion and heat transfer within the flow regime. Also, the dominant effect of heat transfer irreversibility at the lower plates reduces with increasing values of buoyancy force and reduces with heat source.