JOHANNESBURG, Aug 6 (Reuters) - South Africa's Nedbank Group reported an 8% rise in half-year headline earnings on Tuesday, supported by good non-interest revenue (NIR) growth, a lower impairment charge and tight cost control.

The bank's headline earnings, a profit measure, grew to 7.9 billion rand ($427.2 million) in the six months ended June 30, from 7.3 billion rand a year earlier.

NIR increased by 7% to 14.4 billion rand, slightly ahead of the group's full-year guidance of above mid-single-digit growth. This was thanks to good growth in commission and fees, a strong trading performance and higher levels of fair-value gains and equity investment income, the bank said.

Net interest income (NII) - the amount banks earn from loans minus what they pay on deposits - grew marginally by 2% to 20.8 billion rand, below initial management expectations of above mid-single digit growth.

This was as the average interest-earning banking asset grew by 3% and the net interest margin - which shows how much a bank is earning in interest on loans against the amount it pays depositors - fell by 5 basis points to 4.13%.

($1 = 18.4912 rand) (Reporting by Nqobile Dludla; Editing by Jacqueline Wong and Janane Venkatraman)