Mechanical heart valve replacement necessitates long-term warfarin anticoagulation to prevent thromboembolic complications, yet achieving adequate anticoagulation control remains challenging. This study evaluated whether mean platelet volume (MPV) could serve as a biomarker for anticoagulation quality in mechanical heart valve recipients. We conducted a retrospective longitudinal cohort analysis of 370 patients with mechanical heart valves receiving warfarin therapy. MPV and international normalized ratio (INR) were measured at five time points. Time in therapeutic range (TTR) was calculated using the Rosendaal method, with effective anticoagulation defined as TTR ≥60%. Correlations between MPV, INR, and TTR were analyzed overall and stratified by valve type. Only 34.1% of patients achieved effective TTR. Mean MPV was 10.31 ± 1.04 fL and mean INR was 2.79 ± 0.54. No significant correlation was identified between MPV and TTR. However, patients with mean INR ≤3.0 exhibited significantly higher MPV values compared to those with INR >3.0 (10.40 vs 10.13 fL). Subgroup analysis revealed a significant inverse correlation between MPV and INR in mitral valve replacement patients but not in aortic valve replacement patients, with temporal variation across measurement periods. MPV showed poor discriminatory ability for predicting effective TTR. Multivariable regression identified atrial fibrillation, older age, higher platelet counts, and higher hemoglobin as independent predictors of effective TTR, while MPV showed no independent association. In conclusion, MPV does not serve as a reliable biomarker for anticoagulation quality in mechanical heart valve patients. Enhanced anticoagulation management strategies are needed to improve TTR in this high-risk population.