Based on Williams' work and rewritten in action angle variables, a met
hod for the calculation of proper elements is here presented. The aver
aging over the long periodic terms is performed by the semi numerical
method developed by Henrard (1990); no series expansion in eccentricit
y or inclination of the asteroid is used which allows calculating prop
er elements for highly inclined orbits. Conversely, the theory is trun
cated at the first degree in the eccentricity and the inclination of t
he perturbing planets. A few tests about accuracy and consistency are
presented.