  The thermodynamical properties of interacting Bose atoms in a harmonic
potential are studied within the mean-field approximation. For weak
interactions, the quantum statistics is equivalent to an ideal gas in an
effective mean-field potential. The eigenvalue of the Gross-Pitaevskii equation
is identified as the chemical potential of the ideal gas. The condensation
temperature and density profile of atoms are calculated. It is found that the
critical temperature $T_c$ decreases as the interactions increase. Below the
critical point, the condensation fraction exhibits a universal relation of
$N_0/N=1-(T/T_c)^{\gamma}$, with the index $\gamma\approx 2.3$ independent of
the interaction strength, the chemical potential, as well as the frequency of
the confining potential.
