The present invention relates to crystalline forms of the Janus kinase (JAK) inhibitor 3-((3S,4R)-3-methyl-6-(7H-pyrrolo[2,3-d]pyrimidin-4-yl)-1,6-diazaspiro[3.4]octan-1-yl)-3-oxopropanenitrile (Compound A), as well as, compositions thereof, methods of their preparation, methods of use thereof, and methods of quantitation.本發明係有關Janus激酶(JAK)抑制劑3-((3S,4R)-3-甲基-6-(7H-吡咯并[2,3-d]嘧啶-4-基)-1,6-二氮雜螺[3.4]辛烷-1-基)-3-側氧基丙腈(化合物A)之結晶型,及其組成物、其製備方法、其用途、及其定量方法。