Len McCluskey said members had simply been responding to events. The site's owner, Ineos, should have come forward earlier to discuss a survival plan with the union, he said. He said: "I think in all honesty I have to say no, it was not a miscalculation. What happened in Grangemouth was simply a reaction by our membership to given events."
He added: "It was a series of events that took place, that with the benefit of hindsight of course could have been handled a little differently - if the company had come to us much earlier and sat down and wanted to discuss a survival plan."
In the end the union agreed to a pay freeze and other changes to terms to keep the site open. Mr McCluskey also defended Stephen Deans, the Unite official who resigned from his job at Grangemouth earlier this week.
On Wednesday, David Cameron told MPs Mr Deans was a rogue operator who had almost brought the petrochemical industry in Scotland to its knees. But Mr McCluskey described him as a "decent, genuine, gentle man".
Mr Deans, the Unite convener at the plant, was alleged to have used work time to carry out local Labour Party business.
He is expected to be ousted as the chairman of his local Labour Party in Falkirk this weekend. Members are expected to bring a motion of no confidence in his leadership tomorrow.
The local party has been at the centre of a media firestorm for months amid allegations Unite attempted to rig the contest for Labour's next Westminster candidate for Falkirk. The current MP, former Labour politician Eric Joyce, has announced he is standing down at the next General Election, due in 2015.
Yesterday, Downing Street said the Prime Minister's views on allegations Unite intimidated senior managers of the Grangemouth site and their families had not changed, after Mr McCluskey defended the union's tactics.
Mr Cameron has described the accusations as "shocking" and called for a full investigation.
He said while people had the right to protest, nobody had a right to "intimidate, nobody has a right to bully, nobody has a right to threaten people's families, no one has a right to threaten people in their homes and if these things have happened it is very serious and needs to be properly examined."
Mr McCluskey said the tactics were "legal and legitimate". He added: "If a company is engaged in what we believe is an unfair attack on workers and their families and their communities, then the idea faceless directors can disappear to their leafy suburbs and get away with that type of thing is something we think is wrong".
l Mr McCluskey clarified his neutrality on the independence debate yesterday, following claims he was "warming to the case for a yes vote".
In a statement from Unite, he said: "I have always been absolutely clear the question of Scotland's future is one for the Scottish people alone. I don't have a vote and I don't seek one.
"The position of Unite Scotland and the wider Scottish trade union movement is to remain neutral on the referendum issue, but to focus debate on the pressing social and economic issues Scotland will face, whatever its relationship with the rest of the UK.
"Unite's eventual position in the referendum will be a matter exclusively for our Scottish membership and our Scottish regional committee."