The BSE barometer touched an intra-day high of 20,484.28 points at the outset but later trimmed gains as retail investors booked profits during afternoon trade. The Sensex ended the day at 20,286.99, a gain of 80.04 points or 0.40 per cent over Friday's close of 20,206.95 points.
Small-cap and mid-cap shares continued to attract heavy buying interest for the fifth consecutive day. Both the indices ended the year at the all-time closing peaks.
Foreign Institutional Investors were believed to be heavy buyers in front line counters in a bid to push the net asset value of their portfolio at the end of their financial year.
The broader S&P CNX Nifty of the National Stock Exchange also advanced by 58.90 points or 0.97 per cent to 6,138.60 from previous close of 6,079.70.
FIIs were heavy buyers in key shares at the concluding trading session of the year, while local investors continued to build up positions in key counters, market players said.
Refinery shares were in the forefront and closed with smart gains on expectations of petrol price hike in the near future. State-run refining and marketing companies HPCL, Indian Oil and BPCL registered hefty gains.
IT stocks, however, suffered a moderate setback due to fresh selling pressure.