A kind of photonic crystal defect waveguide with rectangular holes which works in the terahertz domain is designed.Firstly,the band-gap property of the triangular air hole photonic crystal is analyzed.A line defect is introduced into the photonic crystal to form the waveguide.The air holes adjacent the defect are set as rectangle.The effect of the rectangular holes size on the band gap and the group velocity of the waveguide is analyzed and the holes size is obtained.The effect of the width of band gap on the defect mode is also studied.Then the defect mode frequency is moved to the target frequency 338 GHz by optimizing.A low group velocity of c0/1543 (c0=3×108 m/s) at the Brillouin boundary is realized by using the photonic crystal defect THz waveguide with rectangular holes.Then its good slow-wave characteristic is proved.