Aerofoils with a sinusoidal leading edge pattern have been the subject of the study during the recent decade due to its considerable effect on the aerodynamics performance. The leading edge variation results in the formation of streamwise counter-rotating vortices which affect the boundary layer development on aerofoils. To understand the influence of the leading edge variation to the boundary layer flow development, the current study was carried out using a flat plate model. A single hot-wire anemometer was used to measure the streamwise velocity at Reynolds number (based on the wavelength of the leading edge patterns) of 3080 corresponding to free-stream velocity of 3 m/s. The results showed that downstream of the troughs of the leading edge pattern, counter-rotating pairs of vortices form and develop along the streamwise direction. Inflection points on the velocity profiles at upwash region imply the appearance of the vortices with mushroom like structure which diffuse further downstream as a consequence of the mixing enhancement and eventually break down into turbulence.