This article proposes a simply structured transverse slot linear array antenna with a low cross-polarization in X band on a quasi-TEM (transverse electromagnetic) mode waveguide. The fabrication technology of this antenna is very simple and suitable for mass production. A center fed linear slot array has been designed and measured. The quasi-TEM wave is propagating in the conventional waveguide with dielectrics at sidewalls. The simulation and the measurement results show that the baffle plates enhance the gain and reduce the beamwidth effectively. The uncertainties of the electric properties of the dielectric and fabrication errors are also discussed.