Like the positron-electron pair production shown in high energy physics, elementary particles can be created and annihilated in phenomena, which means that energy and mass can be exchangeable especially in subatomic physics. However, it should be clear that mass and energy in physics can be converted to each other in some cases but not always equivalent. From the ontological review of the mass-energy equivalence principle in special relativity and the mass defect in nuclear physics with the first principle in 4-D complex space, we can understand how these two distinctive concepts in physics can be interchangeable. Although nuclear force, which makes nucleons, protons and neutrons, bound inside the nucleus, and nuclear interactions, strong and weak interactions, which participate in nuclear fusion and nuclear fission process, have been explained in standard model of particle physics, in which the existence of an intermediate or force-carrying particle (boson) in the interactions has been known for a long time. However, it is not so clear how the intermediate boson makes the interaction possible. For the interactions of subatomic particles, alternatively we can find more agreeable explanation than the one using the intermediate boson in standard model of particle physics. In addition, a brief discussion is followed for the neutrinos in weak interaction and the lifetime of neutron in free space.