In the present work, we report simple and eco-friendly route to synthesise biocomposite doped nanocrystalline MgO. The seed of Hygrophila spinosa T. Anders (HST) plant showing gelly nature in water and contain several minerals, is the chosen dopant (biocomposite). The structural parameters, functional groups, surface morphology, elemental analysis and optical behavior of the prepared samples were evaluated by using X-ray diffraction (XRD), Fourier transform-infra red (FT-IR), high resolution scanning electron microscope (HR-SEM), Energy dispersive X-ray analysis (EDX), Diffuse reflectance spectroscopy (DRS) techniques respectively. The two probe DC-electrical resistance method was used to evaluate the sensitivity factor (Sf) of the prepared samples at different humidity levels. It was found that the appropriate ratio of dopant into MgO extensively modified the sensitivity. Compared to all the other composition of Mg:HS, the sensitivity factor (Sf) of MH3 (0.25:0.75) is found to be higher i.e., 3,300. The MH3 sample also showed good linearity, reproducibility, stability, response and recovery time about 210 ± 5 s and 232 ± 4 s, respectively. The composition, crystallite size and surface area were found to have the significant effects on the sensitivity. Copyright © 2013 American Scientific Publishers.