Recently, there is increase in the use of natural fiber in the reinforcement of polymer composites due to its low production cost, ease of fabrication and enhanced strength. In this study, the effect of NaOH treatment on the tensile properties of coconut fibre for polyester resin transtibial prosthetic socket reinforcement was investigated. The coconut fibres were extracted using hand pulling and water retting method; NaOH treatment was used to mercerize the fibres was carried out 5% to 25% at time interval of between 30Min. to 150Min. The result indicated that the effect of NaOH treatment on the Coconut fibre vary, depending on the concentration of NaOH and the treatment time. It is concluded that treatment of Coconut fibre in 15% NaOH for 150minutes is the best treatment protocol of Coconut fibre for polyester resin reinforcement.