For a good THz waveguide, both low propagating loss and small mode width are usually very important. However, the high ohmic loss of metals and the high absorption loss of dielectric materials result in that it still remains a challenge to obtain the two capabilities at the same time. In this paper, planar dielectric-gap-metal (DGM) waveguides are presented to guide THz wave. According to the dispersion equations of the waveguides, we calculate their mode characteristics by numerical calculation, and we find that THz wave can propagate in the waveguides with low loss and simultaneously subwavelength mode width. When compared with the parallel-plate waveguide, the mode losses of the DGM waveguide can be 1-3 orders of magnitude lower, but the mode widths do not increase. The combination of low propagating loss and subwavelength mode width makes the DGM waveguides particularly useful for many THz applications such as sensing, communication, and imaging.