The present invention relates to polymorphic forms of (S)-1-cyanobutan-2-yl (S)-1-(3-(3-(3-methoxy-4-(oxazol-5-yl)phenyl)ureido)phenyl)ethylcarbamate (Compound 1): ##STR00001## pharmaceutical compositions thereof. The present invention also relates to processes to prepare compound 1 and pharmaceutical compositions thereof. Compound 1 is an IMPDH inhibitor useful in treating IMPDH-mediated diseases such as immune system related diseases, viral diseases, and cancers.