Thermal spacing control using a heater in a magnetic head has been a key technology for achieving lower magnetic spacing. However, spacing change is nonlinear to the heating power, and the relationship between the heating power and the spacing change in low spacing region is very important to clarify. We numerically evaluated spacing change focusing on the effect of heating power. The numerical results were verified by comparing with experimental results. By coupling Reynolds equation and heater-induced pole-tip protrusion (H-PTP), numerical results agreed well with experimental results. Using the numerical model, we found the H-PTP shape difference due to the heat transfer change rarely influences the spacing change, and the height of H-PTP significantly affects the spacing change. Also, we clarified that the nonlinearity is significantly influenced by the lowest spacing and the initial pressure at the maximum H-PTP area. This knowledge is the key for a design of spacing control by a heater.