A novel crystalline precursor for preparing Y3Al5O12 (YAG) nanocrystallite was synthesized via a co-precipitation method using (NH4)2CO3 solution as the precipitator. The precursor was characterized by means of powder X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), thermal gravimetry (TG), differential thermal gravimetry (DTG) and differential scanning calorimetry (DSC), respectively. The empirical chemical formula of the crystalline precursor can be expressed as 5[(NH4)2Al6(CO3)3(OH)14]×9[Y2(CO3)3×3H2O] according to the test results. The measured weight loss of 46.7% of the precursor without consideration of the absorbed water is in accord with the calculated value of 47.5% according to the above empirical formula. The phase-pure YAG nanocrystallites were obtained by calcining the above precursors at 900°C for 2 hours in air atmosphere. Transmission electronic microscopy (TEM) result showed that the particle size of YAG nanocrystallites is 40-80 nm. The mechanism of themal decomposition of the crystalline precursor was also presented.