Forest timber production potential of any site is oftentimes measured quantitatively by site index, which is defined as dominant height of a particular stand at a specified age. A site index was developed for estimating site quality of monoculture Pinus caribaea plantations in southwestern Nigeria using a base age of 25 years. Dominant height data were collected from 60 Temporary Sample Plots (TSPs) of $20{ imes}20m$ in plantations of 15 to 37 years. Linear and non-linear models as been widely applied in quantitative forest measurements were fitted to dominant height-age data and the best site index equation is : $SI=Exp^{(InHd-23.495(A^{-2}-0.04)}$. The site index curves constructed for the three sites (Omo Forest Reserve - OFR (J4), Oluwa Forest Reserve - OLFR and Shasha Forest Reserve - SFR) across the southwestern Nigeria using the chosen equation revealed that a 15 year old Pinus caribaea in the study area attained average dominant heights of 25, 22 and 21 m in OFR (J4), OLFR and SFR respectively. The site index equation and curves proffer veritable insight into better silvicultural options and management practices for the future plantations suitable sites.