We investigate direct conversion of ordinary mode into upper hybrid (UH) waves by pre-existing electron density irregularities during ionospheric heating. The electric field amplitude and the average power absorbed by the excited UH waves are calculated when the irregularity distribution is (i) coherent, and (ii) incoherent with a broad band power spectrum modeled by a drift wave type scaling. For the typical parameters of the experiments, we find that the direct conversion process can be quite efficient.