Mo and Si3N4 powder mixtures in the molar ratio of 1:3 were mechanically activated by high-energy ball milling for 0-100h. No MoSi2 formation was observed during milling. DTA and TGA results showed that the reaction between Mo and Si3N4 enhanced with increase in milling time. The milled powder mixtures were reacted at different temperatures between 1000 and 1400°C for the synthesis of Si 3N4-MoSi2 in situ composite. Complete conversion of Mo in 10-100h milled sample into MoSi2 has been observed after pyrolysis at 1400°C for 1h. In the unmilled (0h) sample lower silicide of Mo i.e., Mo5Si3 has been detected. In the mechanically activated samples, after 10h of milling, no Mo5Si 3 formed after reaction at 1400°C for 1h. The intensities of peaks of MoSi2 in the pyrolysed samples increased with increase in milling time. In the pyrolysed sample both nano- and sub-micron size MoSi 2 particles were observed to be uniformly distributed through out the Si3N4 matrix. © 2004 Elsevier B.V. All rights reserved.