Objectives: (1) to determine serum vitamin 25(OH)D3 level and (2) to determine relation between vitamin 25(OH)D3 level and other factors in patients with knee osteoarthritis at Bach Mai hospital. Patients and Methods: cross-sectional study included 70 patients with osteoarthritis (ACR 1991 criteria for osteoarthritis) at Bach Mai hospital from 7/2012 to 10/2013. Patients haven't been diagnosed knee osteoarthritis and haven't been treated osteoarthritis and osteoporosis before administration. All patients were examined and recorded body mass index (BMI). Knee radiograph ere given scores for global severity of osteoarthritis, using a modification of the scale of Kellgren and Lawrence (range, 0 to 4). Serum concentration of vitamin 25(OH)D3 was measured by the Electrochemiluminescence immunoassay on the Roche Elecsys system (Roche Diagnosis Elecsys). Bone mineral density (BMD) was measured by Dual-energy X-ray absorptiometry (DEXA) test. Result: Study included 70 patients (16 men 23 percent and 54 women -70 percent). (1) Mean serum vitamin 25(OH)D level was 26,01 + or - 6,7ng/ml. There was 67,1 percent patients had serum vitamin 25(OH)D 30ng/ml. (2) The serum vitamin 25(OH)D3 in the group below the age of 65 was 28:7,9ng/ml, (lnd that in the group above the age of 65 was 24,7:5,6ng/ml, p0,05. The serum vitamin 25(OH)D3 in the group having one osteoarthritis knee was 27:6,2ng/ml, and in the group having both osteoarthritis knees was 23,3 + or - 7,3ng/ml (p0,05). The serum vitamin 25(OH)D3 in patients having stage 2 osteoarthritis was higher than in patients having stage 3 (p0,05)knee. There was positive association with femoral neck T-score (r