This paper uses a 3‐D fiber laser Doppler velocimeter (FLDV) to measure the turbulent flow field inside a pancake type combustion chamber of a four‐stroke single cylinder transparent engine. Three components of the instantaneous flow velocity at the central point on a plane 10 mm below the cylinder head are obtained to characterize the flow field before combustion. The experiment employed a filtering method to resolve the high frequency turbulence and low frequency cyclic fluctuation from the instantaneous velocity data. In addition, a temporal autocorrelation coefficient, integral time and length scales, and a normalized power spectral density function are also calculated to characterize the turbulence structure. Variations of parameters, such as compression ratio and engine speed, are carried out in the experiment to study the effects of the design factor and the operation parameter on the characteristics of the in‐cylinder flow field.