SHANGHAI/SINGAPORE, Feb 18 (Reuters) - China's central bank left a key policy rate unchanged as expected when rolling over maturing medium-term loans on Sunday.

The People's Bank of China (PBOC) said it was keeping the rate on 500 billion yuan ($69.51 billion) worth of one-year medium-term lending facility (MLF) loans to some financial institutions unchanged at 2.50% from the previous operation.

In a Reuters poll of 31 market watchers, 22, or 71%, of all respondents expected the central bank to keep the borrowing cost of the one-year MLF loans unchanged on Feb. 18.

With 499 billion yuan worth of MLF loans set to expire this month, the operation resulted a net 1 billion yuan fresh fund injection into the banking system.

The central bank also injected 105 billion yuan through seven-day reverse repos while keeping borrowing cost unchanged at 1.80%, it said in an online statement. ($1 = 7.1929 Chinese yuan) (Reporting by Winni Zhou and Tom Westbrook; Editing by Raju Gopalakrishnan)