In this study, cellulose nanofiber was prepared from WH and used as reinforcement in recycled paper. There are various sources to prepare Cellulose nanofiber however, water hyacinths being used as a source to synthesize cellulose in this study to decrease its devasting effect on water body. Pure cellulose was isolated from water hyacinth with chemical treatments including dilute alkali swelling and bleaching. The chemical composition of water hyacinth was gravimetrically determined and the result demonstrated that cellulose is the largest constituent of the plant (55 %) followed by hemicelluloses (19%) and lignin (14%). The 12 % was related to other extracts like fat, oil, pectin etc. As the FTIR data revealed, that the functional groups corresponding to lignin and hemicelluloses were absent in the FTIR spectra of extracted cellulose. To increase its functionality the prepared cellulose was disintegrated into nanofibers using a simple mechanical treatment. To get more information about the fibrillation of cellulose, 1wt. % water dispersed mechanical treated cellulose light transmittance was measured using UV-Vis spectroscopy. The dispersion showed high light transparency, particularly at a higher wavelength. Hence, from the reasonable light transmittance value of cellulose dispersion, it can be concluded that the aggregated cellulose chains were effectively converted into nanofibers. The synthesized cellulose nanofibers in this research were used as reinforcement in recycled paper. The moisture sensitivity, thermal stability and mechanical strength of recycled paper were significantly improved upon inclusion of cellulose nanofibers as fillers.