Waste materials from plants to soil have been proven as an alternative means of nematode control. A greenhouse study was conducted to investigate the effect of Grapefruit Waste Compost (GWC) on the inhibition of root knot nematode, Meloidogyne javanica, and some possible mechanisms involved in nematode suppression such as C:N ratio, total phenolic component in the tomato roots and leaf chlorophyll contents were measured. Two months before transplanting of tomato seedlings, sterile and non- sterile soil was amended with dried residues of Grapefruit at 3, 5 and 7% (w/w). Sixty days after nematode inoculation, the results showed a direct correlation between increasing GWC and the final population of nematode (Pf). At rate of 7% in non-sterile soil, Pf decreased by 65% compared to non-amended control. In this study one of the nematicidal efficacy of amendments was directly correlated with their nitrogen content and inversely related to their C:N ratios. By increasing GWC, the C: N ratio was lower than non-amended control. The most efficacious against the nematode was when soil was amended with a high GWC ratio (7%). Also, tomato grown in the compost-amended media showed the presence of higher quantity of total phenols than those grown in non-amended soil. One of the possible causes of reduction in final population of M. javanica should be derived from the limitation in nematode penetration rates due to high phenolic component in the root which may also affect on the maturity of the female and decreasing the number of eggs. In this study, no phytotoxicity was observed and soil amendment with GWC increased the leaf chlorophyll content. In overall, the results suggested that inhibition of nematode with GWC relates to complex components and various mechanisms in the soil environment, as well as the biochemical strategy of tomato plant grown in such media.