National Trust director-general Dame Helen Ghosh says the conservation charity has an "open mind" about allowing fracking on its land.
Her comments appear to contradict the Trust's position of a "presumption against" the controversial technology.
Campaigners say the environmental risks of fracturing rock to release gas are too great.
A spokesman says the Trust has not changed its position but will review any new evidence in the future.
Fracking is the process of drilling down into shale rock before a high-pressure water mixture is directed at the rock to release the gas inside.
Water, sand and chemicals are injected into the rock at high pressure which allows the gas to flow out to the head of the well.
In an interview with The Times Newspaper, Dame Helen said: "We all have yet to see what the surface environmental impact of fracking is and when we have seen it we would reach a view."
She added: "We are waiting for the evidence. We have an open mind."
However, the Trust's position on fracking says: "We have a presumption against fracking on our land because natural gas is a fossil gas.
"The mining process also gives rise to potential environmental and landscape impacts."
It goes on to say: "Whilst the use of natural gas might buy time to develop secure, renewable alternative energy sources, it also risks distracting us from focusing on the development of these and on the need for us all to concentrate on using less energy in the first place.
A Trust spokesman said: "Our position is a presumption against fracking on our land.
"It doesn't mean that in time that won't change - if the environmental and visual impacts are negligible and it is part of a strategy to move to a low carbon economy."
The Trust adopted its position of a presumption against the gas/oil extraction technique during the summer's anti-fracking protests at Balcombe, West Sussex, where energy company Caudrilla planned to carry out test drilling.
Earlier this year, the charity's director of conservation, Peter Nixon, described fracking as "fool's gold".
He said: "If it takes our eyes off the ball from the need for energy efficiency and a long-term sustainable source of renewable energy, it will be fool's gold."
The spokesman told BBC News: "Shale gas is a fossil fuel [that] may prove a fool's gold and distract attention from a shift to renewables.
"The environmental and visual impacts are also unproven, and this is what concerns many people. We want to see more evidence on what this impact will be."