The present research aims at establishing an accurate and reliable methodology to estimate the heat quantity generated during the rotation of spindle and to evaluate the characteristics of the spindles employed for various types of machining. The technique proposed here basically depends on the accurate measurement of the change in the rota-tional speed of spindle during its free run. A new method is presented in the present report to measure the rotational speed of spindle accurately in a rotational speed range up to 100000 rpm by applying a reflective sensor system. The method developed here was applied to estimate the heat quantity of various types of spindle. It was clarified through experiments that the surface roughness of the spindle rotor affects the heat quantity generated, and also that the coefficients of the thermal expansion of the rotor and spindle housing affect the clearance between the two and consequently the heat genera-tion. The effect of air stirring by the bolt holes is also discussed.