They said investors dumped energy shares while stocks of shipping firms were also down after they reported worse-than-expected quarterly earning results.
The Stock Exchange of Thailand (SET) composite index shed 5.48 points to close at 700.75 and the blue chip SET 50 index fell 4.81 points to 491.93.
Turnover stood at 1.8 billion shares worth 9.7 billion baht (242 million dollars) traded. Losers outnumbered gainers 204 to 86, with 113 stocks unchanged.
"The market was down due to profit-taking. Investors turned sellers after the stock posted strong gains last week," said Sukbir Khanijoh, an analyst at Trinity Securities.
"Apart from selling of energy shares, we saw selling of shipping firms after they reported weak earning results," Sukbir said.
The Thai baht closed on Monday at 40.98-41.07 to one dollar, compared with 40.90-92 on Friday. Against the euro, the Thai unit closed at 48.31-67, against Friday's 48.82-87.
Among major shares, PTT fell 2.00 baht to 228.00 while its subsidiary PTT Exploration and Production shed 4.00 to 440.00.
Thailand's biggest commercial lender, Bangkok Bank, shed 2.00 to 101.00 and the country's fourth biggest bank Siam Commercial Bank fell 0.50 to 49.50.
Thai Airways International declined 0.50 to 39.50.
Major Thai shipping firm Precious Shipping lost 1.75 to 38.75.