The 30-share barometer, which had gained 24.15 points in the previous choppy session, moved down by 22.97 points, or 0.13 per cent, to 17,749.87 points in early trade.
The wide-based Nifty of the National Stock Exchange declined by 6.30 points, or 0.12 per cent, to 5,383.90 points.
Brokers said investors turned cautious and reduced their positions due to a weak trend on the Asian markets after rating agency Moody's cut the debt ratings and outlooks of several European countries.
Healthcare, capital goods and oil and gas stocks were under pressure, which led to decline in the Sensex.
In the Asian region, the Hong Kong's Hang Seng index shed 0.03 per cent, while Japan's Nikkei was quoted 0.15 per cent higher in early trade today.