Abstract: Objective To investigate the correlation between the vitamin D level and mean platelet volume (MPV). Methods 420 patients were collected. According to the vitamin D level, the patients were allocated to Group 1 (vitamin D <10ng/mL), Group 2 (vitamin D, 10-20ng/mL), Group 3 (vitamin D >20ng/mL), respectively. The vitamin D level and mean platelet volume in all patients were detected. Results The vitamin D level in Group 1 (7.7±1.9ng/mL) was lower than that in Group 2 (14.3±1.7 ng/mL, P<0.001) and Group 3 (24.2±5.9 ng/mL, P<0.001). The MPV in Group 3 (7.3±1.5 fL) was lower than that in Group 1 (8.3±1.7 fL, P<0.001) and Group 2 (8.0±1.4 fL, P =0.006). Linear regression analysis showed that low levels of vitamin D (r2=0.337，P<0.001) was independently associated with increased MPV. Conclusion The vitamin D level is closely related with MPV. The vitamin D deficiency would results in a higher MPV.