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The availability of decanter cake (DC) and palm kernel expeller (PKE) from palm oil processing remains high owing to the major revenue earned from palm oil industry. Rearing black soldier fly larvae (BSFL) using DC and PKE as substrate to produce frass could solve the indiscriminate disposal of these wastes. Rearing BSFL was carried out by hatching the black soldier fly eggs in the trays size of 65 cm length x 37 cm width x 15 cm depth laid with processed soybean as starter diet for a week. After seven days, the DC and PKE were fed to the BSFL as main substrates and continued for another 38 days until the BSFL reached to pupae stage. Temperatures of the ambient and rearing substrate were monitored daily for 45 days. The frass of BSFL was analyzed for some selected physico-chemical properties and germination test to ascertain the quality of the BSFL frass to be used as organic fertilizer. Colour of the BSFL frass, electrical conductivity (EC), organic matter (OM), pH, total carbon (C), nitrogen (N), C/N ratio, ammonium, and nitrate contents were determined using standard procedures. Frass was produced at 45 days of rearing BSFL after which the larvae were turn to pupae. Frass consists of excretions, substrate residues, and shed exoskeletons of the BSFL. The pH of BSFL frass was 7.41, whereas the EC (1.21 mS/cm) was in the range of permissible level for its uses as organic fertilizer. The BSFL frass contain 1.68% total N, 8870.19 mg kg−1 ammonium, and 5453.20 mg kg−1 nitrate which were higher than those of initial mixture of the feeding substrate due to the decomposition process of the organic substrates which is related to the lower content of total OM (45.23%) and C (26.23%). Germination of white radish (test crop) on a distilled water and the different extractions of BSFL frass were not significantly different suggested that the frass produced from BSFL fed on DC and PKE had no toxic effects on plant. Decanter cake and PKE could be used as rearing substrate for BSFL to produce frass based on the appropriate values of pH, EC, total N, ammonium, nitrate, total organic matter and total organic C, and C/N ratio.