The organisation is Scotland's largest conservation charity and manages more than 128 sites, including battlefields and properties such as Falkland Palace in Fife. It suffered a loss of £1.2m in 2005/06 and visitor numbers dropped by 11,000.
A 30-day consultation between staff and management was announced last Friday. Alan Denney, national officer for Scotland for the trade union Prospect, which represents 400 of the 500 staff, claimed the majority of cuts will be made to front-line staff.
"There's nothing wrong with organisations like the NTS running in a business-like manner, but they do have to remember their raison d'être is to preserve and protect," he said.
A spokesman for the charity said its financial problems were due to the decline in visitors, combined with an increase in the cost of maintaining properties.
"We are looking at proposals to make the charity more sustainable and could not put a figure on the number of jobs that will go while the consultation is going on.
"We have not announced any job cuts yet, but have spoken to around 70 staff whose jobs could be affected."