A traditional Greek tragedy, according to Aristotle, has to be a tightly unified play based on a single action and featuring a single protagonist. In addition, this tragic hero suffers a downfall because of some error of judgement or frailty rather than a vice or some form of depravity. Thus it is that Macbeth for example can be said to be a tragedy because of Macbeth's ambition and how it brings his downfall.
However, when we think of these categories, Antigone does not exactly fit the mould of such a Greek tragedy. First of all, Antigone herself does not occupy much of the play. She is a character who is desposed of quickly by Creon. Secondly, it is hard to detect what her tragic flaw is, unless it is her resolute belief that the Gods and family should be honoured above the law of the land. She indeed does suffer a terrible end for this adherence to a higher law, and is undoubtedly a tragic character in the way that she laments about never being able to be a wife, but at the same time she doesn't seem to fit the description of a tragic hero or heroine according to Aristotle.