Black soldier fly larvae (BSFL) are known to convert organic wastes into useful biomass, of which the composition depends on the substrate. It is of interest whether feed protein can be sustainably obtained from waste materials by feeding them to BSFL. This study aimed to convert rice straw and duck manure into BSFL biomass for conversion of waste into animal feed. The growth parameters of BSFL fed with pure fermented rice straw, pure duck manure, and an equal parts mixture of the two as well as its nutritional composition was determined. The larvae’s efficiency to consume and convert the different substrates was also evaluated. Results showed that BSFL fed with duck manure had significantly higher average individual weight of 0.0619±0.004 g, followed by mixture of duck manure and rice straw (0.0614±0.001 g), while those fed solely with rice straw did not accumulate the same biomass (0.0415±0.002 g). Correlations were also made for mass-length, mass-width, and length-width. Mass-length connection was the most reliable correlation (r = 0.732). The harvested BSFL protein was the highest for those fed with rice straw at 34.62%. Feed conversion ratio ranging from 3.71 to 11.3 was achieved for the substrates used. The availability of the waste substrate in large quantities coupled with efficient biomass conversion makes BSFL a sustainable organic matter converter primarily useful as additive to animal feed.