<p>Background: MicroRNAs (miRNAs) are implicated in various biological processes including anticoagulation. However, the modulation of miRNA by pharmacological intervention such as warfarin treatment in patients receiving warfarin has not been disclosed yet. The aim of this study work was to assess the effect of warfarin drug on expression level of mir-133a-3p in patients with mechanical heart valve replacement.<br /> Methods: In this research, the expression level of miRNA-133a-3p was analyzed in Peripheral Blood Mononuclear cells (PBMCs) from mechanical valve replacement patients who had received warfarin for at least 3 months continuously. Quantitative RT-PCR method was used for this assay.<br /> Results: Our findings indicated a significant diffrence between the rate of miR-133a-3p expression in individuals receiving warfarin and the control group (p&lt;0.01). There was also a statistically significant difference in miR-133a-3p expression in patients with different ages (p&lt;0.05) suggesting that the rate of miR-133a-3p expression in persons receiving warfarin is related to age. However, other variables like warfarin dose, International Normalized Ratio (INR), gender, and Body Mass Index (BMI) were not significantly effective on the miR-133a-3p experssion rate in individuals receving warfarin.&nbsp;<br /> Conclusion: Based on our results, it can be concluded that miR-133a-3p is involved in the coagulation pathway. The recent result indicates that warfarin affects the expression of miR-133a. This expression may be potentially important for treatment by anticoagulants. Awareness of the time course of miRNA expression profile can improve efficiency of response to warfarin.</p>