A theoretical model for carbon dioxide (CO2) migration in tilted aquifers with groundwater flow is presented to evaluate the injection of CO2 into a geological formation. Capillary force in the flow of two immiscible fluids in a porous medium creates a saturation transition zone, where the saturation changes gradually. A vertical equilibrium assumption is employed to solve for the capillary pressure. Initially we verify our analytical model without slope and incoming ground water. Next the effects of sloped angle and an incoming ground water are studied. The asymmetrical distribution is fully incorporated in our analysis presented in this work, which provides essential information for CO2 injection period and reservoir capacity. In the limiting case of no sloped stratum and no incoming groundwater flow as well as no transition zone, the results for our analysis compare very well with prior works. For the stratum with a slope angle, CO2 will migrate further in the upper side of CO2 injection point. The incoming underground water helps CO2 move further on the up-dip side of the CO2 injection point where CO2 flow direction is the same as the incoming underground water. The existence of a critical velocity when the incoming CO2 at the injection point will only move to the up-dip side is established. In this work, for the first time we account for the injection velocity and the saturation transition zone as well as a sloped incoming groundwater flow. (C) 2014 Elsevier Ltd. All rights reserved.