Successful net-shape sintering offers a significant advantage for producing large or complicated products. Porous Si3N4 ceramics with very low shrinkage were developed, in the present investigation, by the addition of a small amount of carbon. Carbon powders (1-5 vol%) of two types, with different mean particle sizes (13 nm and 5 mum), were added to alpha -Si3N4-5 wt% Y2O3 powders. SiC nanoparticles formed through reaction of the added carbon with SiO2 on the Si3N4 surface or with the Si3N4 particles themselves. Such reaction-formed SiC nanoparticles apparently had an effective reinforcing effect, as in nanocomposites. Sintered Si3N4 porous ceramics with a high porosity of 50%-60%, a very small linear shrinkage of similar to2%-3%, and a strength of similar to 100 MPa were obtained.