Cissus quadrangularis biodegradable natural cellulosic fibers comprehensively characterized to assess their potential as reinforcing materials in polymer composites. Initially, the Cissus quadrangularis fibers were chemically treated with 5% Sodium hydroxide (NaOH) and 5% magnesium carbonate (MgCO3) to improvise the properties of the fiber. The mechanical test result shows that chemically treated 5% MgCO3 fiber show that 4% and 24% improved tensile strength compared to NaOH and untreated ones. The cellulose crystallinity of the treated fiber got increased as the amorphous constituents removed. Further, 5% MgCO3 treatment removed a larger amount of amorphous hemicellulose, lignin, and other impurities present on the fiber surface. Secondly, the composites were fabricated at different combination of MgCO3 (5%, 10%, 15%), plasticizer (5%, 7.5%, 10%), and fiber volume (20%, 25% and 30%) with L9 Taguchi orthogonal array approach. Based on the results, 5% MgCO3, 5% plasticizer, and 30% fiber volume showed significant improvement in Young's modulus, tensile, and flexural strength of 8%, 27%, and 16% respectively. Moreover, there was no notable improvement observed on impact strength for both treated (15.91 KJ/m2) and untreated (13.98 KJ/m2) fiber. The scanning electron microscopy (SEM) micrographs used to examine the interface bonding between fiber and the matrix.