The effects of fluid flow and solidification conditions on the columnar-equiaxed transition are investigated. The ratio, R, of heat flux at dendrite tip to that at dendrite stem is introduced as a parameter which characterizes the columnar-equiaxed transition (For R>1, columnar grains grow, whereas for R<1, equiaxed grains grow). This heat flux ratio, R, is calculated for Fe-0.5%C alloy under varying condition by using the finite element method. The heat flux ratio thus calculated indicates, in good agreement with previously reported observation, that the columnar-equiaxed transition occurs when the temperature gradient decreases to about 5°C/cm under natural convective flow faster than 0.3cm/sec, and columnar grains come to grow again as the temperature gradient further decreases to less than 1°C/cm. For flow slower than 0.3cm/sec, only columnar grains are shown to grow.