Spatial and temporal variations of b values are studied by means of 624 earthquakes in the Andaman-Nicobar Islands region during a five-year period preceding the giant shock on December 26, 2004. Sliding time and space windows containing 50 events are used. Temporal variations reveal two significant drops in b value which coincide with the occurrence of two large shocks (Ms ≥ 7.0) towards the end of 2002 and with the Mw = 9.0 event in 2004. The spatial distribution shows accumulation of stresses (low b) around the epicentres of the 2002 and 2004 events and an event in North Andaman. The area of high stress around the 2004 epicenter extends in NNW-SSE direction over an area about 450 km long.