Abstract: Recombinant human interleukin-11 (rhIL-11) was identified by high-performance liquid chromatography and electrospray ionization quadrupole time-of-flight mass spectrometry (HPLC-ESI-Q-TOF-MS). The molecular mass of rhIL-11 was measured by ESI-Q-TOF-MS. The trypsin digested peptides of rhIL-11 at 37 ℃ over night were measured and then two double charged peptides ( m/z 664.45, 609.40) were selected to measure its amino acid sequence by HPLC-ESI-Q-TOF-MS/MS. Search database with the measured amino acid sequence to identify rhIL-11. The measured molecular mass of rhIL-11 was 19 047.38 u and as well as the theoretical value is 19 047.29 u. The two peptides’ sequences measured by HPLC-ESI-Q-TOF-MS/MS were Glu-Pro-Glu-Leu-Gly-Thr-Leu and Leu-Asp-Ser-Thr-Val-Leu, which was hIL-11 as a result searched in MASCOT database.