Abstract: The influence of the fouling process of CaCO₃ on heat transfer performance was investigated experimentally. Hard water self-made was used as working fluid. More parameters such as temperature, hardness, and fluid velocity etc were measured in the experiment to verify their effect on the fouling process. It was showed that the surface and fluid temperatures were very important to promote fouling process on heat transfer surface. Also, the hardness of water can quicken the fouling forming on the heat transfer surface.