The reason for diamagnetism is that when a substance is in an external magnetic field, the external magnetic field changes the motion of the electron orbit (more precisely, the orbital domain) of the substance. When a magnetic field B is applied, a magnetic force F will be generated for the moving electron (charge Q): F = qv× B. This force changes the centripetal force to the electron, making the orbital motion of the electron either accelerate or decelerate. The velocity of the electron thus changes, which in turn changes its orbital magnetic moment in the opposite direction to the applied magnetic field.

Consider two electron orbital domains: one moves clockwise and the other moves counterclockwise. An external magnetic field entering the page direction will increase the centripetal force of clockwise rotating electrons and increase their magnetic moment in the page direction. The same external magnetic field will reduce the centripetal force of the electron rotating counterclockwise and its magnetic moment entering the page direction. Both of these changes contend with the external magnetic field entering the page direction. However, for most everyday substances, the magnetic moment induced by external magnetic field is very small, so the net effect will be a repulsive force.

All substances will respond to external magnetic fields to varying degrees. But for materials with other magnetic properties (such as ferromagnetism and paramagnetism), diamagnetism can be completely ignored. Those substances that only show diamagnetism or show diamagnetism to a great extent are called diamagnetism materials or diamagnetism particles. Those materials that are considered diamagnetic are usually considered nonmagnetic by non-physicists. They include water, DNA, most organic compounds, such as petroleum and some plastics, and metals, such as mercury (element), gold and bismuth.