An hydraulic jet mill is disclosed in which a high pressure line and a low pressure line are introduced into at least one jet (or nozzle). The jet is configured such that there is an orifice at the forward end of the jet and the cross sectional area of the interior of the rearward portion of the jet is significantly greater than the cross sectional area of the interior of the forward portion of the jet. There is an incompressible liquid such as water in the high pressure line and a material to be milled in a slurry with an incompressible liquid such as water in the low pressure line. The high pressure line is introduced into the rearward portion of the jet and the low pressure line is introduced through a venturi into the forward portion of the jet. The contents of the high pressure line and the low pressure line mix and are forced out of the orifice at high velocity. The material is milled when it strikes either an impingement plate or an opposed second jet.