Using a modified version of the mass vector model, Hassan (1997), we explain that the particle mass decreases (increases) if the particle in attractive (repulsive) field. The concepts of the free rest mass and the rest mass in a field are introduced. We conclude that the mass of a particle in a field changes according to the rule , where is the rest mass of the particle in the field. This result is consistent with the existence of the Higgs field through the space and it causes existing particles to acquire mass. The confined theory is explained. General mass equations of a particle and of a composite particle in terms of different four forces of nature are obtained. Some applications of the general mass equation of composite particle (nucleus) are presented and discussed.