The interaction of plane layer of unmagnetized plasma with terahertz waves is investigated in this paper. The reflection coefficient of terahertz wave of conducting plane covered with unmagnetized plasma is computed by using analytic method and finite difference time domain (FDTD) method, respectively. The FDTD method used in here is the piecewise linear current density recursive convolution (PLCDRC) FDTD methodology. The analytic method used in this paper is the Wentzel-Kramer-Brillouin (WKB) method which often be employed when the properties of the medium vary slowly on a scale comparable to the wavelength. The results illustrate that the attenuation of terahertz wave through a plasma slab is a decreasing function of EM wave frequency, and a decreasing function of plasma collision frequency. Besides, we discover that the absorption tends to zero as the EM-wave frequency approaches terahertz. The agreement of results between PLCDRC-FDTD and WKB methods is shown.