Fe1.5Mn1.5Si undergoes a paramagnetic to a ferromagnetic transition with true long range ordering. The critical behavior of the system near the transition temperature has been studied by analyzing dc magnetization data with the help of the Arrot plot. The present work is focused to answer an important question, whether the nucleation of domains starts at a temperature close to TC or different from it. The enhancement technique in ac susceptibility helps us to determine the domain nucleation temperature. The domain nucleation temperature is found to be close to TC obtained from scaling analysis. At the same temperature, the imaginary component of the ac susceptibility (c¢¢) also shows a peak.