The Chinese yuan hit a record high against US dollar on Thursday, reaching the CNY 6.4 level for the first time in 17 years, according to the China Foreign Exchange Trading System. The People\'s Bank of China set the yuan-dollar central parity rate at CNY 6. 3991 compared with Wednesday\'s rate of CNY 6.4167. It was the strongest level since China unified exchange rates in 1993. The yuan strengtened against the dollar after the Federal Reserve pledged on Tuesday to keep intrest rates at a record low for at least another two years to boost the US economy. China\'s central bank fixes the central parity rate, an official reference for daily trading, every morning, and allows the currency to rise or fall by 0. 5 percent from the central parity rate. The yuan-dollar central parity rate is based on a weighted average of prices before the opening of the market each trading day.