Centerline porosity in horizontally cast steel plates has been examined analytically. The presence and extent of porosity has been calculated on the basis of heat flow in the system and interdendritic fluid flow. The calculated distribution of porosity was found to agree closely with measured values reported in the literature. Several solidification parameters have been proposed to indicate the onset of porosity. The present investigation shows that the parameter G/R1/2 is the best of the parameters suggested, where G andR are the centerline temperature gradient and growth velocity, respectively.