Fluorescent nanodiamonds (NDs) are hard, nontoxic, have high melting point and possess physical and chemical stability. In this paper, the NDs with nitrogen vacancy (NV) centres have been studied with steady-state and time-resolved photoluminescence spectroscopy. The broad fluorescence spectrum spans in the visible-near IR region (from 500 nm to 900 nm) with presence of prominent characteristic peaks of neutral and negatively charged NVs. The fluorescence decay of bulk NV centres shows multi-exponential decay with different lifetimes at two excitation wavelengths. When brought in contact with the polystyrene microspheres, the NV centres exhibit whispering gallery modes (WGMs) built upon the background spectrum. The fluorescence decay of NV centres in presence of the microcavity gives evidence of inhibition of the decay rate. Simulations from the finite element method show evanescent field coupling. The results have been explained in terms of the Purcell's effect as well as the slow light that results due to the interference of the ballistic and circulating lights in the microcavity. © 2021 Elsevier B.V.