A three-layer slab waveguide with left-handed material is investigated. Through Maxwell equations, a dispersion equation for TM oscillating guided modes is obtained and some mode dispersion curves are plotted, respectively. It is found find that they have an abnormal dispersion property and two modes degenerancy near cutoff. Besides, the normalized power flux of TM oscillating guided modes in the slab waveguide is studied. The results reveal their power fluxes have positive value, negative value and zero. The positive or negative power flux means energy in waveguide travels forward or backward. But, zero power flux implies all energy is stored in the slab waveguide.