We calculate the ground-state energy and the effective spin Lande g factor (g*) of a two-dimensional electron gas in strong magnetic fields using a variational Monte Carlo method and many-body trial wave functions composed of the Jastrow correlation factor and the Laughlin-type functions. We find that g* increases with decreasing of electron density and also shows increasing behavior as the filling factor nu decreases in the region of 1<nu<2. At nu=1.0, g* is estimated to be about 11 for a dimensionless parameter r(s)=1.33, as compared to the measured value of 7.3, while considering impurity and phonon scatterings gives a better agreement of g*=9.