In the present research investigation, a novel natural treatment technique consists of Sapindus mukorossi and citric acid was used to treat the fibers as an alternative to conventional chemical treatments that can have adverse effects on both the environment and human health. The fibers were treated with natural solution for varying time intervals of 24, 48, 72, 96 and 120 h, and reinforced with poly-lactic acid using injection moulding machine. The thermal, mechanical and moisture absorption properties of developed naturally treated biocomposites were examined through FTIR spectroscopy, thermogravimetric analysis, and mechanical analysis to explore the optimal treatment period. The optimal treatment period for the fibers was determined to be 72 h, during which overall properties of biocomposite were observed highest. The biocomposites fabricated with 72 h of treatment exhibited 16.8 % improvement in tensile modulus and 22.8 % increment in flexural modulus as compared to untreated fiber biocomposites. Furthermore, the highest levels of water absorption and impact strength were achieved through treatment duration of 96 h and 48 h, resulting in significant improvements of 63.2 % and 22.2 %, respectively. The assessment of surface characteristics was carried out through the use of optical microscopy and scanning electron microscopy (SEM).