* Jan-Aug sales up 43 pct on year to 353,129 unit
* Stock falls 2.4 pct at noon, have lost 30 pct this year (Adds quotes and shares price)
HONG KONG, Sept 7 (Reuters) - Chinese car and battery maker BYD Co Ltd, backed by Warren Buffett, said its August vehicle sales fell 5.9 percent from July on keen competition amid slowing growth in the world's largest auto market.
BYD, which is 10 percent owned by Buffett's Berkshire Hathaway Inc, sold 31,069 vehicles in August, down 19 percent from the same month last year, the company said in an email to Reuters on Tuesday.
The homegrown automaker underperformed rivals as overall car sales in China rose 59 percent in August on year.
"Overall car sales in China were weak in July and August, and some automakers cut prices," said Paul Lin, a spokesman at BYD Auto, the car manufacturing unit of the group.
Vehicle sales for the first eight months of 2010 rose 43.4 percent to 353,129 units, representing about 59 percent of BYD's revised sales target of 600,000 for the whole of 2010.
"We will watch for September and October, which are traditionally golden months for auto sales," Lin said.
In late August, BYD launched two new models, the M6 and L3, which could help boost sales for the rest of the year, the spokesman said.
"We plan to launch the S6 by the end of the year," he added.
Shares of BYD ended the morning session down 2.4 percent at HK$48.05, underperforming the Hang Seng Index's marginal gain.
China's auto sales are returning to a more normal pace from the breakneck growth levels last year and the beginning of 2010, when government's stimulus measures boosted sales.
BYD cut its 2010 sales target by 25 percent in July and its stock has lost about 30 percent so far this year on concern that high inventories with dealers could continue to pressure sales. (Reporting by Alison Leung; Editing by Chris Lewis)