The real-time technique of time-averaged holographic interferometry is applied to determine the natural frequencies and corresponding mode shapes of clamped rectangular plates with straight narrow slits, and the effect of the lengths, positions and inclination angles of slits on the natural frequencies and corresponding mode shapes is investigated. Four types of slit locations for a slit parallel with respect to the sides of a plate and a slit having various inclination angles with respect to the longer sides are considered. The natural frequencies obtained experimentally are expressed in terms of a dimensionless frequency parameter, and the results are shown graphically as a function of dimensionless slit length and inclination angle. Several interesting findings are made.