Heat flux to a plane water jet impinging on a high temperature flat surface is measured at its stagnation point by a transient method. In the film and nucleate boiling regions, experiments are also made by a steady-state method. The results obtained by the two methods substantially agree. In the film boiling region, no liquid-solid contact is found. At the lower limit of the film boiling, incipience of liquid-solid contact is detected. The boiling curve obtained is not always monotonous from the minimum heat flux to the maximum, but has a 'shoulder' of nearly constant heat flux within a certain wall temperature range at some conditions. An analytical solution to the film boiling of an impinging liquid jet is performed by applying the two-phase boundary layer theory.