Single longitudinal mode operation of a Rhodamine-6G dye laser by using both of a tunung wedge and a Michelson mode selector was investigated. Stable single mode operation was achieved when the distance between the jet and the cavity end mirror was equal to the path difference between the two arms of the Michelson selector. Spatial hole burning effect in a Fabry-Perot type dye laser could be compensated at this condition. Single mode of the dye laser was investigated by using a scanning Fabry-Perot interferometer with 1.4 GHz free spectral range. Single mode output power of the dye laser was measured as a function of the $Ar^+$ laser pumping power. Threshold of 0.9 W pumping power for single mode operation was lower by half than that of single mode operation by using etalons. Output power of 180 mW at a wavelength of 590nm could be obtained with 3.8 W $Ar^+$ laser pumping power which was higher than that in case of etalons. The slope efficiency of the single mode output was 6%.