Demands for power devices are expanding with the electrification of automobiles. Power devices are soldered using Ni-P films prepared by an electrochemical method to withstand required high currents. However, cracks are inevitably induced in Ni-P films during high mounting temperatures. Ni-P films play an important role as a solder joint material and the P concentration is a critical factor that affects the mechanical properties of Ni-P films. Therefore, in this work, we focused on the P concentration and tried to prepare Ni-P films without cracks induced by high heat treatments. Ni-P films with different P concentration were formed on an Al substrate by an electroplating using a Watt bath and then investigated the relationship between the P concentration and the generation of cracks by X-ray diffraction and thermal expansion analyses. By the heat treatment at 300 °C or higher, Ni-P compounds mainly composed of Ni₃P was formed in the Ni-P film with P concentration of 8 wt.% or more and the film shrank by 0.11%. In contrast, the 2 wt.% or less P- Ni film did not cause the formation of Ni₃P and little shrinkage was observed even after the high treatment at 400 °C. The obtained results indicate that the phase transition to Ni-P compounds by the heat treatment is responsible for the mechanical disintegration, that is, the P concentration in the film is closely related to the generation of cracks. We propose the method for suppressing cracks by controlling the P concentration in the Ni film.