Abstract: The insulation or dielectric circuit of electrical machines constitutes their weakest member, as copper and steel, the other chief constituents can withstand elevated temperatures. It is therefore, imperative to qualify electrical insulating materials thermally by determining their insulation class. In this study, twelve sample varieties of Nigerian wood species-Abeza, Afara, Agbagum, Aper, Bomba, Danta, Gameliana, Iroko, Mahogany, Obeche, Opepe and White wood were experimented with to determine their insulation classification. The samples were cut into definite dimensions and weighed. Each wood species was made into two samples; one sample was left in its ordinary state while, the other was impregnated with an insulating varnish. Both samples were subjected to a heat-run in a sealed industrial oven. The insulation resistance of each sample was measured at regular temperature intervals, until the sample burns out. The measured values of weight, insulation resistance and temperature are shown in tables. Curves were plotted to show the variation of insulation resistance with temperature. From the experiments, eight of the wood species-Danta, Afara, Obeche, Mahogany, Opepe, Abeza, Aper and Iroko can be used for class H insulation while, the other four species Gameliana, Agbagum, Bomba and White wood are suitable for class F insulation.