We present radio observations and modelling of one of the nearest and brightest Type IIP supernova SN 2004dj exploded in the galaxy NGC 2403 at a distance of ~ 3.5 MPc. Our observations span over a frequency range of 0.24 - 43 GHz at ages from 1.12 days to 12 years since the discovery date 2004 July 31. We model the radio light curves and spectra with the standard mini-shell model and derive the parameters of explosion. We estimate the mass-loss rate of the progenitor star as $\dot{M}$ ~ 6.56 × 10^{-6} M _{sun}yr^{-1} . We calculate the radio spectral indices using 1.06, 1.40, 5.00 and 8.46 GHz flux density measurements at multiple epochs. The spectral indices are smaller than the expected optically thin spectral index values for an extended period predominantly at higher frequencies. We explain this trend as a signature of electron cooling happening at the supernova shock in the plateau phase of the supernova. We estimate the cooling timescales for inverse Compton cooling and synchrotron cooling and find that inverse Compton cooling is the dominant cooling process.