Lightweight concrete is the way to reduce the weight as well as deflection in concrete members without affecting its properties. Many of the researches are in progress to find a substitute for this lightweight material. In this project, we would like to take the naturally available fibre named sisal fibre and bamboo as partial replacement material. The influence of sisal fibres on the strength of concrete is taken as the main objective of this experimental study. The addition of natural fibre to the lightweight concrete will enhance the various strength parameters like flexural strength, compressive strength, and increase the ductile behaviour. In the present work, it is aimed to investigate the mechanical properties of lightweight concrete with a replacement of sisal fibre for cement and bamboo as a replacement in coarse aggregate in different percentages. The compressive strength, flexural strength, deflection of the beam is studied with consideration of M25 concrete specimens. Totally 36 number of 500 x 100 x 100mm flexural member cast and tested. It is recommended up to 5% replacement of coarse aggregate with bamboo and 5% addition of sisal fibres with cement provide at M25 grade of concrete gives the optimum increases of strength values. The test results indicated that the sisal fibres were effective in improving the strength of lightweight concrete.