This invention relates a crystalline form of N-[4-[2-(2-amino-4,7-dihydro-4-oxo-1H-pyrrolo[2,3-d]pyrimidin-5-yl)ethyl]benzoyl]-L-glutamic acid, pemetrexed diacid, and to a process for the preparation thereof.本發明提供N-[4-[2-(2-胺基-4,7-二氫-4-側氧基-1H-吡咯并[2,3-d]嘧啶-5-基)乙基]苄醯基]-L-麩胺酸,即培美曲塞二酸(pemetrexed diacid)之結晶型式,及其製備方法。