In this study, the effect of pilot bit placement on the performance of an OFDM system with comb-type based channel estimator was investigated. Computer simulations were used for this purpose. Least sequares algorithm is employed to estimate the channel at pilot sub-carriers and low-pass interpolation to estimate the channel at data subcarriers. The pilot ratio (ratio of data bits to pilot bits) that gave the lowest bit error rate was determined as the Optimum Pilot Ratio (OPR) and spacing between the pilot sub-carriers as (Δf) OPR. Then the relationship between (Δf)OPR and channels' coherence bandwidths was investigated using four channels with different frequency selectivity. Simulation results show that, taking spectrum efficiency and channel estimation accuracy into account, two or three pilot subcarrier within coherence bandwidth at 0.9 correlation is likely to be an optimum choice. © 2011 IEEE.