The study focused on lactic acid (LA) production using rice straw as waste product using plant-originated Lactobacillus rhamnosus PN04 in different treatment of heat. Straw has been treated with heat before L. rhamnosus PN04 leading to the pH decrease gradually to 4.68 compared to the absence of L. rhamnosus PN04 (5.91) and heat untreated straw cases. By Thin layer chromatography (TLC) and Infrared spectroscopy (IR) analysis, there was strong evidence for LA production in this condition. The result has driven a cheap method to produce LA for pharmaceutical field, food and chemical industry.