An experimental study has been performed to obtain the effect of relative velocity between droplet and environmental gas on the evaporation of a n-heptane free droplet of 180 $mu$m in diameter flying in a hot and normal pressure air flow. Measurement of droplet diameter and velocity was conducted in a series of time by an electrically controlled optical system. From the experimental results, an empirical equation associated with the relation between evaporation rate constant and relative velocity was obtained.