The Court of Appeal said David Morris and Helen Steel will be allowed to appeal "some" of the points on which, at an earlier hearing, they were found to have libelled the fast food chain.
In a 300-page document, the three appeal judges also cut McDonald's £60,000 damages award by £20,000.
Mr Morris, 44, a former postman, and Ms Steel, 33, a former gardener, were ordered in June 1997 to pay £60,000 damages after a 314-day trial - the longest libel case in English legal history.
Mr Justice Bell ruled that the company had been libelled by most of the allegations in a London Greenpeace campaign leaflet entitled What's Wrong with McDonald's?.
But he found the leaflet was true when it accused McDonald's of paying low wages to its workers, being responsible for cruelty to some of the animals used in its food products and exploiting children in advertising campaigns.
Libel law as 'censorship'
During the appeal hearing Mr Morris told the court that McDonald's had never applied for its damages or costs or for an injunction banning further publication of the leaflet.
He said the use of libel laws by multinational corporations was a form of censorship and called for a change in the law to protect campaigning organisations being sued by them.
The pair argued that Mr Justice Bell erred in not clearing them overall after he ruled that some of the points they made were correct.
Ms Steel told the judges during the hearing that the leaflet was reporting what other environmental organisations and health experts were saying about McDonald's and its products.
She said that neither of them had written or printed the leaflet and the High Court's ruling that their involvement in London Greenpeace showed "joint enterprise" contravened the European Convention of Human Rights guaranteeing the right of freedom of association.
Northern Ireland
Scotland
Wales
England
McDonald's
McSpotlight: Libel support group
The BBC is not responsible for the content of external internet sites.
Next steps for peace
Blairs' surprise over baby
Bowled over by Lord's
Beef row 'compromise' under fire
Hamilton 'would sell mother'
Industry misses new trains target
Quins fightback shocks Cardiff
(From Sport)
Vodafone takeover battle heats up
(From Business)
IRA ceasefire challenge rejected
Thousands celebrate Asian culture
Christie could get two-year ban
(From Sport)
Colleagues remember Compo
(From Entertainment)
Mother pleads for baby's return
Toys withdrawn in E.coli health scare
Nurses role set to expand
(From Health)
Israeli PM's plane in accident
More lottery cash for grassroots
Pro-lifers plan shock launch
Double killer gets life
Cold 'cure' comes one step closer
(From Health)
Straw on trial over jury reform
(From UK Politics)
Tatchell calls for rights probe into Mugabe
Ex-spy stays out in the cold
Blair warns Livingstone
(From UK Politics)
Smear equipment `misses cancers'
(From Health)
Boyzone star gets in Christmas spirit
(From Entertainment)
Fake bubbly warning
Murder jury hears dead girl's diary
Germ warfare fiasco revealed
(From UK Politics)
Blair babe triggers tabloid frenzy
Tourists shot by mistake
A new look for News Online