Science and Education Publishing International Journal of Physics 2333-4886 2 3 2014 06 22 The Rest Mass of a Particle in a Field and a General Mass Equation 86 95 EN Mohamed Ahmed Hassan Department of Mathematics, Faculty of Science, Ain-Shams University, Cairo, Egypt IJP2014234 10.12691/ijp-2-3-4 2014 05 26 2014 06 07 2014 06 22 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.