The effects of quantum confinement on electron spin g-factor have been investigated in lattice-matched GaAs/AlGaAs quantum wells (QWs) and strained InGaAs/GaAs QWs. We demonstrate how the spin precession frequency changes depending on the GaAs well width and the indium fraction in In x Ga 1-x As well by time-resolved photoluminescence measurements under a high magnetic field. The g-factor values obtained by the spin precession measurements are compared with precise calculation method proposed by Ivchenko. The calculated results agree with our experimental results in lattice matched QWs. In addition to this calculation, we have examined a simplified k·p perturbation calculation. It is found that the k·p calculation exhibits good agreement with the experimental trends both in the unstrained and strained QW systems.