The boiling process is one of the most effective ways for heat transfer enhancement. During the spray cooling and drop impingement to a hot surface, the incipient bubbles grow and coalescence inside a thin liquid film. This paper presents high-speed top-view visualization together with a stereomicroscope of drop dynamics and evolution of bubbles formed inside the water drop impacted to the substrate with a temperature varied from 90 to 150 C. The nucleate and transition boiling regimes are considered. Stereomicroscope has a large depth of field, allowing capturing many features of the interaction of bubbles and their dynamics in a thin layer of liquid. Top-view visualization made it possible to estimate the bubble size and diameter, and accurately determine the cause and time of rupture of the liquid film. Histograms of bubble sizes were plotted using data over the entire area of the thin liquid film at different time points. The average diameter of the bubbles is shown to increase over time up to 160-200 mu; m at the moment of liquid droplet breaks. This value is found to be independent on the substrate temperature, while the growth rate of bubbles in a liquid droplet strongly depends on the heat flux. The maximum and minimum contact diameters of the droplet decrease with increasing substrate temperature.