The main objective of this study was to manufacture black onions using different methods, and to analyze the color and physiological and antioxidant activities of their water extracts. Black onions were produced via high-temperature aging (70, 80, and $90^{circ}C$). At $90^{circ}C$, onions with different weights (large, 90-L; medium, 90-M; and small, 90-S) and small onions with different appearances (whole, 90-S-W; cut, 90-S-C; and peeled, 90-S-P) were also produced. The colorimetry measurements showed that the extract from the 90-S-C black onion had the highest ${Delta}E$ value among all the extracts from all the black onion groups. The extract from the 90-S-C black onion had a 3.3 times higher total phenol content than that from the $70^{circ}C$ black onion extract (11.62 and 3.51 mg/g, respectively). The DPPH radical scavenging activities of the extracts from 90-S-W, 90-S-C, and 90-S-P were higher than those of the other black onion extracts, and the 90-S-C black onion extract had the highest level of 48.3% (at the concentration of 10 mg/ml). The 90-S-C black onion extract also had the highest level of antioxidant activities (ABTS radical scavenging activity, 0.84 mg AA eq/100 g, and nitrite scavenging ability, 49.8%). These results clearly demonstrated that 90-S-C was the best among the conditions used and that the water extract of the 90-S-C black onions had a significantly higher level of antioxidant activities than the extracts from the other black onions.