Patients with myeloproliferative neoplasm (MPN) could have laboratory aspirin resistance and then increasing dose of aspirin from once daily to twice daily regimen is suggested. However, it is not routinely recommended to perform platelet function testing to determine aspirin resistance in MPN patients. Moreover, it is not known whether increasing dose of aspirin would always correct aspirin resistance and significantly prevent the thrombotic events in MPN patients. Therefore, this study aims to compare the efficacy of once daily versus twice daily aspirin in high-risk MPN patients with aspirin resistance. MPN patients with laboratory aspirin resistance will be included in this prospective randomized study and platelet function testing will be repeated at one and six months later. Clinical thrombosis and side effect from aspirin will be recorded for at least 2 years after intervention.