A laser micromachining technique that employs etchant jet for thermochemical wet etching of metallic channels is introduced In this method, the etchant supplies chemicals for thermochemical reaction and laser beam delivered through an optical fiber provides the thermal energy. For the generation of etchant jet, a nozzle with a gap control function is utilized. By using this method, microchannels with a triangular cross-sectional profile and a depth ranging from 100 to $300{mu}m$ and a width ranging from 100 to $200{mu}m$ are fabricated on stainless steel 304. Since the etchant is supplied in the form of a liquid jet, it is expected that this technique can be applied for the fabrication of microchannels on industry scale workpiece. The variation of channel size and shape with respect to process parameters is examined. Also, the potential application to the fabrication of metallic microchannels with high-aspect-ratio is investigated.