The vote was initially due to be held in July, but street violence and political infighting have forced it to be postponed three times.
An inconclusive result in June elections forced the run-off.
Guineans has been subjected to 52 years of authoritarian rule since it gained independence from France.
Last weekend, the two rival presidential candidates – Alpha Conde and Cellou Dalein Diallo – appealed for calm following two days of violence in the capital, Conakry.
The pair had earlier accused each other’s supporters of inciting violence.
The run-off was scheduled for this weekend, but election chief Gen Toumany Sangare said it had been cancelled due to logistical problems.
The UN and US had both urged Guinea to set a new date as soon as possible to avoid instability.
Mr Diallo is seen as the favourite to win the run-off. He took 44% of the votes in the first round – and claims he was denied overall victory by fraud.
Guinea is the world’s largest exporter of the aluminium ore bauxite. It also has important deposits of iron ore, but it remains one of the poorest countries in West Africa.
Source: BBC