Objectives To explore the correlation of level of serum 25-hydroxyvitamin D with pulmonary function in adult with asthma. Methods Patients were divided into Asthmatic group(n=62)and Control group (n=28). The Asthmatic group was further divided into Mild Group (n=6), Moderate Group (n=13) and Severe Group (n=43). Serum levels of 25-hy?droxyvitamin D [25(OH)D], denoted as 25(OH) Vit D was detected by ELISA. Pulmonary function indicators,including FVC (forced vital capacity) , FEV1 (forced expiratory volume in 1 second) , FEV1%predicted , and FEV1/FVC%were deter?mined by a pulmonary function testing device. General profiles such as medical history, age and height as well as serum VitD levels were compared between subgroups of the asthmatic groups and between two genders. Serum levels of 25 (OH) VitD were compared between asthmatic group and control group while its correlation with FEV1%predicted were calculated in all three sub asthmatic groups. Results There was no significant difference in medical history, age, height and the 25(OH) VitD levels between male and female participants. Serum 25(OH) Vit D level was significantly lower in the asthmatic patient group [(29.69±20.45) nmol/L] compared to that in control group [(75.16±4.06) nmol/L] (P<0.05). It was significantly lower in severe sub group than those in the mild and moderate sub groups. The differences were both statistically significant ( P<0.05). There were positive correlations between serum 25(OH) Vit D levels and FEV1%predicted ( P<0.05) in all sub asth?matic groups. Conclusion Vitamin D deficiency is highly prevalent in asthmatic patients, and there is a strong correlation between 25(OH) Vit D asthma severity as well as between lung function.