Experimental investigations of process emissions from atomic layer deposition (ALD) of Al2O3 are accomplished under various temperatures and purge times to understand its environmental sustainability performance. About 93% of Trimethylaluminum (TMA) is found flowing through ALD system without deposition. 2–9 × 104 of ultrafine nanoparticles containing 51.9 ± 4.6% of C, 16.6 ± 0.9% of Al, 31.4 ± 4.1% of O are generated during each cycle of reactions. 0.34–0.38 cm3 of CH4 (25 °C, 1 atm), which takes up 45–51% of C contained in TMA is produced simultaneously. The concentration of nanoparticles drops with the increase of purge time. CH4 also has a trend of decreasing but acts more complex with the largest emission at a short purge time. Compared with temperature, which has limited effects on reactants, purge time changes the time of reaction as well as the degree of gas phase mixing, and therefore greatly influences ALD emissions.