The present invention specially relates to the crystalline form of (4-Nitrobenzyl (4R,5S,6S)-(3-{(3S,5S)-5-[(dimethylamino)carbonyl]-1-[(4-nitrophenoxy)car- bonxyl]pyrrolidin-3-yl}thio-6-[(1R)-1-hydorxyethyl]-4-methyl-7-oxo-1-azabi- cyclo[3.2.0].hept-2-ene-2-carboxylate) of compound Formula I as well as an improved process for preparation of meropenem trihydrate of compound Formula II ##STR00001## wherein PNB represents a P-nitro benzyl group and PNZ represents a P-nitrobenzyloxycarbonyl group.