Abstract: The objectives of the study were to evaluate the effects of calcium and manganese supplementation during oil palm frond fermentation by Phanerochaete chrysosporium on in vitro digestibility and rumen fluid characteristics. This research used a Completely Randomized Design comprising two factors as treatments and each treatment was repeated three times. The factors were Ca dose (1.000, 2.000 and 3.000 ppm) and Mn dose (50, 100 and 150 ppm). The data were analyzed using analysis of variance to measure the differences among treatments; Duncan's Multiple Range Test was used to determine the significance of differences. The parameters measured were as follows: digestibility of dry matter (%), organic matter (%), Neutral Detergent Fibre (NDF) (%), Acid Detergent Fibre (ADF) (%), cellulose (%) and hemicellulose (%). The rumen fluid characteristics considered included the following: pH and the concentrations of NH3 (mM) and Volatile Fatty Acid (VFA) (mM). The results indicated interactions between Ca and Mn in relation to the digestibility of dry matter, organic matter, NDF, ADF, cellulose, pH, NH3 and VFA concentration of rumen fluid. The optimal combination of the minerals used were 2.000 ppm of Mn and 150 ppm of Ca; these values yielded the highest values of digestibility of dry matter (41.914%), organic matter (40.990%), NDF (66.429%), ADF (64.396%) and cellulose (68.524%) and the highest concentration of VFA (117.302 mM).