This study measured sap flow (SF) using Heat Ratio Method (HRM) to estimate the transpiration of two 14 years old oil palms for 29 days. Plant water potential (Ψ_frond) was also observed with a psychrometer. Environmental factors such as solar radiation (Qs), vapour pressure deficit (VPD), rainfall (RF), and soil moisture (SM) were also gauged to quantify their contribution to oil palm water flux. Results showed that SF increased after sunrise, peaked after midday, reached a maximum of 386.77-457.79 cm3 hr–1, and declined thereafter. Environmental factors and Ψ_frond explained 76.59%-80.02% variation in quarterhourly sap flux. The contribution of environment variables on SF followed the order of Qs > VPD > SM > RF. The Ψ_frond was opposite with SF and at the lowest point after SF reached the peak for 30-45 min. Minimum values of Ψ_frond on palms No. 1 and 2 were -0.84 and -0.78 MPa, respectively. The contribution of environment variables on Ψ_frond was less than 24%, with the highest coming from Qs and VPD. Daily stand transpiration of oil palm was at 0.82-1.66 mm day–1 which comprised about 30% of stand evapotranspiration. Hence, Qs and VPD showed a significant role in affecting oil palm transpiration in this study.