AbstractPrecise optical evaluation of electron and hole g ‐factors in single InAlAs quantum dot was performed by using the optical pumping of nuclear magnetic field and its bistable behavior. The measurement is based on the fact that the induced nuclear field is effective only on electrons and can compensate the external magnetic field. The experimental results indicate that the effective electron g ‐factor can change in large magnitude including the sign in a controllable manner. Therefore, the method will be useful for the spin manipulation experiments such as photon‐spin qubit conversion. In addition, the in‐plane electron and hole g ‐factors were measured in Voigt geometry. This is the first report of individual g ‐factors of electron and hole in single InAlAs/AlGaAs quantum dots, which have been reported so far by only a few studies about the exciton g ‐factor. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)