* CSI300 -0.5 pct; SSEC -0.4 pct; HSI +1.0 pct
* Chinese carriers slump on yuan devaluation, but exporters jump
* Excitement in state sector reform not sustainable - trader
SHANGHAI, Aug 11 (Reuters) - China's surprise decision to allow its currency to fall hit the shares of airlines and oil on Tuesday, threatening to boost their fuel costs and dragging stock market indexes lower.
Exporters surged, however, on expectations that the currency move would improve their competitiveness.
The central bank described the near 2 percent devaluation as a "one-off", based on a new way of managing the exchange rate that better reflected market forces, but economists said the timing suggested it was also aimed at helping exporters and the sluggish economy.
The CSI300 index fell 0.5 percent to 4,064.79 points by the end of the morning session, while the Shanghai Composite Index lost 0.4 percent to 3,912.86 points.
"Yuan depreciation is apparently bad for importers and good for exporters," said David Dai, Shanghai-based investment director at Nanhai Fund Management Co Ltd.
"The excitement over state sector reforms (see in recent sessions) is not sustainable. The market will be volatile."
Import-related sectors weighted on China's main indexes, which jumped over 4 percent on Monday on hopes of state sector restructuring.
Top carriers China Eastern, China Southern and Air China all slumped more than 4 percent in Shanghai, and tumbled over 10 percent in Hong Kong.
Oil giants PetroChina and Sinopec lost over 1 percent in Shanghai.
Exporters such as trading house Shanghai Materials Trading Co and Garment makers Luthai Textile Co surged by their 10 percent daily limit by midday.
"There has been a gap between the strength of the yuan and China's economic fundamentals," said Du Changchun, analyst at Northeast Securities. "The recent trade data was ugly."
Traders generally brushed aside the market impact of a possible change in China's top securities regulator, Xiao Gang. Sources told Reuters that China's ruling Communist Party has begun looking for an eventual replacement for Xiao, who faces internal criticism over his handling of this year's boom and bust in Chinese stock prices.
Stocks rose in Hong Kong, with the Hang Seng index added 1.0 percent to 24,755.51 points, and the Hong Kong China Enterprises Index gainin 1.5 percent to 11,461.84. (Reporting by Samuel Shen and Pete Sweeney; Editing by Kim Coghill)