The dependence of the effective g-factor (g*) on the electron density and the Landau level filling factor nu are studied for a two-dimensional electron gas in strong magnetic fields. The trial many-body wavefunctions are constructed by multiplying the Jastrow correlation factor to the Laughlin-like wavefunctions, then, the ground state energy are calculated using a variational Monte Carlo method for 1 < nu < 2. For a dimensionless parameter r(s) = 1.33, g* is estimated to be about 11 at nu = 1.0, while the measured value is 7.3. Considering the level broadening due to impurity and phonon scatterings, a better agreement of g* = 9 is found.