Britain's largest fundraising charity, whose annual accounts, published last week, revealed income had risen to £477m, is also introducing formal appraisals for trustees.
The changes will make the organisation compliant with the Code of Governance for the Voluntary and Community Sector for the first time.
The charity's governance arrangements have remained largely unaltered since it was formed by a merger in 2002.
Harpal Kumar, its chief executive, said "everyone felt 20 trustees was too many".
Eight trustees each from the Cancer Research Campaign and the Imperial Cancer Research Fund, plus four others, were appointed at the time of the merger.
"The reasons for these appointments have disappeared into history," said Kumar. The reduction, which should be completed by next month, was being achieved by "terms ending and natural attrition", he said.
Other changes include establishing a nomination and governance committee to oversee the recruitment of new trustees by open advertising, restructuring some committees and reviewing the chair's role at least once every three years. Kumar said many of the changes had already been implemented but had not been written down.
Judith Rich, a governance expert, supported the cut in trustee numbers. "You get a much more focused board," she said.
She described trustee appraisals as "an unusual and interesting development" but said it was not enough to increase the frequency of council meetings from four to six a year. "If you asked a FTSE 100 company to meet as infrequently as that, they would throw their hands up in horror," she said.
Gill Edelman, former chair of the Governance Hub, said smaller boards were often more effective for decision-making.
"It is hard to construct and manage a discussion to a collective conclusion with lots of people," she said.
Cancer Research UK's annual accounts also revealed that fundraising income grew by 2 per cent to £420m in 2007/08 and total income by £9m to £477m.