Permanent-magnet generators can be very efficient, do not require excition. However, the voltage regulation of Permanent-magnet generators became large in their over load conditions, because the magnet field cannot be controlled. It is preferable that the voltage regulation factor of generators has a small numerical value. In this paper, I discuss the voltage regulation of IPM (Interior Permanent-Magnet) generator.The IPM generator has two-layers of the permanent-magnet embedded in the rotor. The voltage regulation performances of the IPM generator are calculated using extended to cover the effect of various parameters on the voltage regulation performances. Experimental results of the IPM generators which confirm the analyses are also given. In this experimental result, the voltage regulation factor is 1.5% at the resistive load.