Focusing on uncertain aerodynamic coefficients which characterize the thrust and torques, an adaptive controller for a four-rotor mini helicopter is designed based on Immersion and Invariance (I&I) approach. In order to deal with the complex multivariable parameter estimation, a relatively simple model parameterized with respect to a few unknown parameters is developed. For the estimation of those parameters, the I&I approach is employed and the design parameters of the resulting estimator are easy to adjust. With the estimates of the parameters, several tracking controllers are designed based on the backstepping methodology. Numerical simulation illustrates the robustness of the controller against the parametric uncertainties and external disturbances.