This paper investigates the hydrothermal response of plant fiber-reinforced-polyester composites (PFRC). Experimental methods were used to determine the mechanical properties of PFRC (bamboo, raffia and coconut fiber composites) through the use of monasanto tensometer testing machine. All the samples were chemically modified through the use of 12.5g of sodium hydroxide. The ultimate tensile strengths and moduli of raffia, bamboo and coconut fiber -reinforced polyester composites were computed when the composites were subjected to 20, 40, 60 and 100oC temperature and also soaked in water for periods of 4,8,12 and 24 hours. Numerical and micro-soft excel graphics were used to model the tensile responses of the PFRCs. From the analyses, the ultimate tensile strengths and moduli of raffia, bamboo and coconut composites for 24 hours at 100oc are 4.3, 5.8, 7.7MPa and 0.1, 0.06, and 0.11GPa respectively.