HTC says its consolidated sales for November fell to T$30.94bn (£654m) from T$38.48bn a year ago and T$44.11bn in October 2011.
"It shows how volatile the mobile device market is," said John Strand, founder of Danish mobile industry consultancy Strand Consult.
Since Apple entered the smartphone market with its iPhone in 2007, old market leaders Nokia and Research In Motion have quickly lost ground. RIM warned last week on its sales and profits.
"Apple fans want the new iPhone model. HTC customers do not have the same desire for the next HTC phone. They need to reinvent themselves. It is difficult for consumers to see what they get if they buy a new HTC phone," Strand said.
HTC warned on November 23 its revenues would not grow in the fourth quarter, shocking a market used to double- and even triple-digit percentage growth rates.
Shares in the Taiwanese company more than trebled in the 14 months to April, while sales grew four-fold in a year and a half, as consumers snapped up its innovative phones with their distinctive large clock numerals.
But the stock has slumped 62 per cent over the past six months. It closed up 2.1 per cent at T$458 before the sales figures were published.
HTC had 10.8 per cent smartphone market share in the third quarter, according to research firm IDC, behind Samsung, Apple and Nokia.