Investigations to overcome the brittle response and limiting post-yield energy absorption of concrete led to the development of fibre reinforced concrete using discrete fibres within the concrete mass. Out of the commonly used fibres, easily available low cost natural fibres are renewable source materials. Though these fibres are ecologically advantageous, they have some limitations such as lower durability and lesser strength. But recent research provides several treatment processes to enhance the durability of natural fibres. In this paper, the durability of natural fibres such as coconut coir and sugarcane bagasse has been reported by conducting an experimental investigation. This investigation includes two parts. The first part focuses on the determination of mechanical strength properties such as compressive, tensile, modulus of rupture and flexural properties of natural fibre reinforced concrete specimens once every 3 months for a period for 2 years under alternate wetting and drying conditions. Gain or loss in strength of composite concrete at 9 intervals were computed and are reported here. The second part covers the microstructural properties of fresh natural fibres in as received condition and natural fibres reacted with concrete under accelerated curing conditions for two years. SEM and EDAC test results are discussed.