This article discusses Ushul al-Fiqh methodology of a famous intellectual from school of Zahiri, namely Ibnu Hazm through his work al-IhkÄm fi Uṣūl al-AhkÄm. Eventhough this school of thought cannot exist and be solid followed by muslims as like Sunni and Shi’i schools of thought, the basis of methodology in formulating Islamic law can be investigated clearly especially in a book of al-IhkÄm fi Uṣūl al-AhkÄm written by Ibnu Hazm. By using text approach, this article analyzes Ibnu Hazm’s thought which is stressing on a method of deciding Islamic law based on nash Al-Qur’an and Al-Hadits. Therefore, the thought rejects every method based on reasoning as like al-Qiyas, al-Istihsan, al-Maslahah al-Mursalah etc. Moreover, it also rejects istidlal with dalil al-khitab approach which is still included in reasoning into texts of al-Qur`an dan al-Sunnah. The findings show that first, ijtihad formulation based on Ibnu Hazm is as follows: (a) it must be conducted based on three pillars of Islamic law sources namely Al-Qur’an, Sunnah dan Ijma’ ; (b) if it cannot find zahir nash in the three pillars, Ibnu Hazm conducts ijtihad methods namely al-dalÄ«l which is devided into two namely dalil taking from nash Al-Qur’an and Sunnah, dalil taking from ijma’. Second, the implication of Ibnu Hazm’s ijtihad methodology into the construction of Islamic law is that the building of Islamic law is Al-Qur’an and Sunnah, not others.