  We present year-scale morphological variations of the Crab Nebula revealed by
Chandra X-ray observatory. Observations have been performed about every 1.7
years over the 3 years from its launch. The variations are clearly recognized
at two sites: the torus and the southern jet. The torus, which had been
steadily expanding until 1.7 years ago, now appears to have shrunk in the
latest observation. Additionally, the circular structures seen to the northeast
of the torus have decayed into several arcs. On the other hand, the southern
jet shows the growth of its overall kinked-structure. We discuss the nature of
these variations in terms of the pulsar wind nebula (PWN) mechanism.
