  The ratio of the leptonic charged kaon decays $R_K= \Gamma(K^{\pm} \to
e^{\pm}\nu) / \Gamma(K^{\pm} \to \mu^{\pm}\nu)$ is sensitive to the structure
of the week interactions and can be precisely calculated within the Standard
Model. Presence of New Physics can introduce a shift on its value of the order
of a percent. The NA62 experiment at CERN SPS used data from a dedicated run in
2007 to perform a measurement of this ratio and probe the lepton universality.
The data analysis technique and the final results are presented.
