The effect of isothermal heat treatment on the microstructure of semi-solid AZ91D Ce magnesium alloy was investigated and the ideal globular and near-globular microstructure was obtained. The results show that the rare earth Ce can hider aggregating and associating of the primary solid particles, inhibit growing of the solid particles and form fine well-distributed round semi-solid microstructure. During higher isothermal heat treatment temperature, there is more liquid phase due to the stronger movability of atoms and faster melting rate of solid particles. The size of solid particles is decreased first and then increased. The size of solid particles is mainly decided by melting in initial stage of isothermal heat treatment. Therefore, the size of solid particles is decreased. With further increasing of the isothermal heat treatment time, the size of solid particles is mainly decided by coarsening mechanism and Ostwald ripening and then the size of solid particles is increased.