It has been suggested that feeding a soft diet could possibly inhibit normal development of the masticatory function. However, the consequences of such changes in the alimentary habits have yet to be fully clarified. Therefore, the aim of this study was to determine whether a soft diet prevents the development of masticatory function and whether a critical period for programming the masticatory system exists. To examine these hypotheses, we used a three-dimensional jaw-movement tracking device and jaw muscle electromyography (EMG) to analyse masticatory function changes in mice. Jcl:ICR mice were divided into three groups, with the normal group fed a hard diet, the hypofunctional group fed a soft diet, and the rehabilitation group first fed a soft diet that was then changed to a hard diet. Our results showed that the excursion and duration of late-closing phase (occlusal phase) of the chewing cycle and EMG activity in the masseter muscle were not only reduced in the hypofunctional but also in the rehabilitation group as compared with the normal group. These results suggest that optimisation of the chewing pattern and acquisition of appropriate masticatory function are impeded by feeding a soft diet during the animal's growth period and that no catch-up effect of the masticatory function is observed when there is a prolonged period of time prior to changing the diet from soft to hard. In conclusion, masticatory function can only be fully developed through a learning process such as exposure to chewing various kinds of foods with different food textures.