Renewable energy development could be an appropriate method to tackle the problems caused by fossil fuels. In this regard, an Oscillating Water Column (OWC) with a scale of 1:10 has been studied in this research. Sloshing is an unwanted phenomenon in OWC. Therefore in order to study sloshing, A Novel experimental approach for measuring free surface profile inside OWC by considering the effect of wave characteristics and sloshing was investigated experimentally in this paper. Eight wave gauges were installed at the top of the converter to study the hydrodynamic behaviour. The results showed that an increase in frequency raises the possibility of sloshing. However, up to the frequency of 0.37, the TFS (Top Free Surface) angle increase is insignificant. As frequency approaches 0.37, suddenly TFS angle rises, and sloshing occurs at frequencies of 0.37–0.80. Also, dead volumes are formed on TFS (some positive and negative volumes formed on free surface which cancel each other). Therefore if TFS angle is more than 3.85°, the sloshing is happened, and the converter's performance decreases severely. In the best conditions, efficiency and power are 53.7% and 192.2 W, respectively, and in sloshing area, efficiency and power are 8.3% and 35 W, respectively.