A machinability of porous Si3N4 ceramics composed of columnar β-Si3N4 grains was evaluated by turning and milling with cemented carbide cutting tools to investigate a possibility of application of the porous Si3N4 to machinable ceramic use. The porosity and the flexural strength of specimens were about 43% and 270MPa, respectively. The machinings were carried out with a cemented carbide throw-away tip of K-10 grade. The condition of turning was cutting speeds of 50 to 150m/min, feed rates of 0.05 to 0.40mm/rev, and a cutting depth of 0.5mm, and that of milling was cutting speeds of 20 to 200m/min, feed rates of 0.05 to 0.20mm/rev and a cutting depth of 0.5mm. Machinability was judged from the maximum surface roughness, Rmax, after machining; it was defined that a material was machinable when Rmax was below 10μm. In the case of turning, the Rmax increased with increasing cutting speed and feed rate. Rmax below 10μm was obtained under conditions that satisfied both a cutting speed of 50m/min and feed rates below 0.1mm/rev. In the case of milling, although the Rmax increased with increasing feed rate, serious cutting speed dependence of Rmax could not be seen. Rmax values below 10μm were obtained at feed rates below 0.1mm/rev, independent of cutting speed.