This study was conducted to investigate the effects of two different periods (i.e wet and dry) and five different oil palm materials [i.e. tenera (DxP) and clonal palms (i.e. PL68, PL104, PL106 and PL127)] on leaf carbon isotope discrimination (Δ), leaf gas exchange parameters such as net photosynthesis (PN), stomatal conductance (gs ), transpiration rate (Tr ) ), intercellular CO2 concentration (ci) and instantaneous water use efficiency (WUE) and their relationships, in different oil palm materials cultivated on peat. Leaf gas exchange of 8-year old DxP and clonal palms were determined during dry and wet periods which were also analysed for leaf Δ. Results showed that there were significant differences in PN, gs and Tr between different periods. The gs values decreased by 54% during dry period compared to the wet period. This reduction in gs led to a reduction of 59% and 44% in PN and Tr , respectively. The interaction effect between oil palm materials and different periods had a significant influence on the leaf Δ values. Leaf Δ in clonal palm PL106 was less discriminated (23.74±0.42‰) compared to the D×P palm during dry season (27.92±0.70‰). A negative relationship was observed between leaf Δ with instantaneous WUE and leaf ∆ was found to be highly correlated with ci in oil palm during the wet period.