The present invention relates to a process for obtaining pure aprepitant substantially free of undesired diastereomeric isomer, namely 5-[2(S)-[I (RS)- [3,5-bis(trifluoromethyl)-phenyl)ethoxy]-3-(S)-(4-fluorophenyl)-morpholin-4-yl- methyl]-3,4-dihydro-2H-1,2,4-triazol-3-one. The present invention further provides an improved process for preparation aprepitant crystalline form II. The present invention also relates to novel amorphous form of aprepitant, process for its preparation and to a pharmaceutical composition comprising it. The present invention further relates to aprepitant having mean particle size of less than about 11.5 microns, process for its preparation and to a pharmaceutical composition comprising it. Thus, for example, aprepitant having the content of diastereomeric impurity of 1.1 % is dissolved in ethyl acetate at 700C, the solution is concentrated to half the initial volume by distilling off ethyl acetate, and the resulting solid is collected at 0 - 50C to give pure aprepitant substantially free of diastereomeric impurity.