A coupled analysis involving natural convection, thermal balance, and microstructural evolution that take place in the solidification process of a hypoeutectic gray cast iron is presented in this work. The microstructural formulation used in this study includes classical models of primary-austenite and eutectic (gray and white) transformations. The influence of both natural convection and heat-transfer conditions on the thermal-microstructural response is particularly assessed in a simple cylindrical casting system. The evolutions of temperature and different microstructural variables are compared and validated with available experimental measurements.