Fuel film formed in the spray-piston or cylinder wall impingement plays a critical role in engine performance and emissions. In this paper, the fuel film formation and the relevant film characteristics resulting from the liquid spray impinging on a flat plate were investigated in a constant volume combustion vessel by Refractive Index Matching (RIM) technique. The liquid film thickness was firstly calibrated with two different proportional mixtures (5% n-dodecane and 95% n-heptane; 10% n-dodecane and 90% n-heptane by volume) pumped out from a precise syringe to achieve an accurate calibration. After calibration, n-heptane fuel from a side-mounted single-hole diesel injector was then injected on a roughened glass with the same optical setup. The ambient temperature and the plate temperature are set to 423 K with the fuel temperature of 363 K. The effects of various ambient density (14.8, 22.8, and 30.0 kg/m3) and injector pressure (120 and 150 MPa) on the liquid film properties were studied. The analysis of film formation contain the spatial distribution and time-resolved evolution of fuel film thickness, the film wetted area, and the film mass at the different operating conditions. The experimental results indicated that the fuel film behaviors are significantly influenced by the injection pressure but show insignificant difference at the relatively high ambient density. The larger local film thickness distributed near the impingement point region. The local film thickness decreased with the ambient density and injection pressure.