In this study,a case of mycoplasma pneumonia and arthritis in Holstein calves was diagnosed by clinical history,necropsy,histopathology,PCR,gene amplification and sequencing methods. The affected calves were emaciated,with significantly enlarged wrist,knee and tarsus, and respiratory difficulties with a duration of 15 d. At necropsy,serous cellulose arthritis and periarticular tis,local abscess,bronchial lobar interstitial pneumonia,small bleeding spots on the renal surface. Histopathology was seen in chronic hyperplastic interstitial pneumonia,serous cellulose purulent arthritis and periarticular tis. Calf joint solution was collected for 72 h in typical "fried egg"-like mycoplasma colonies. The genome of lung tissue and joint fluid was extracted, and the target band of about 1 500 bp was amplified with mycoplasma universal primers. After sequencing it was compared in the GenBank database,which had the highest homology with Hubei Mycoplasmabovis reference strain,and the homology rate was above 99%.