The presented study deals with the isolation of microorganisms able to produce cellulases which convert microcrystalline cellulose (MCC) to nanocellulose fibers (NCFs) in critical conditions. This conduction, namely, could be defined as the most potent isolate capable of producing such cellulases and has the ability to convert the fibers to nanoscale in situ conditions without any need for chemical or mechanical treatment. The isolates were affirmed that the priorities for the election of cellulolytic isolates are fungi. Three isolates offered good stability in comparison with the other isolates that included bacteria and actinomycetes. Aspergillus flavus isolated from the rhizosphere of Vicia faba plant and identified morphologically and genetically, and was recorded in gene bank with accession number ON428526. This isolate produced NCFs with a length 96±4.3 nm and a diameter 22 ± 3.8 nm, as well as with high crystallinity and thermal stability. The physiochemical characterizations of the cellulosic fiber produced by the investigated isolate (Aspergillus flavus) affirmed that the CNFs had unique features since; they showed excellent stability and homogeneity. This bioconversion applies the produced fibers to biomedical applications such as drug delivery.