Shiv Sena chief Uddhav Thackeray said Tuesday evening that he decided to join hands with the BJP because of the party's changed way of dealing with its allies.
Mr Thackeray was speaking to Sena workers at his residence in Mumbai.
On Monday, Shiv Sena and BJP, the sparring ruling allies, announced that they would contest the coming Lok Sabha and Maharashtra Assembly elections together.
"I experienced the change in the way they treat people, hence I decided to join hands with the BJP," Mr Thackeray said.
He also reiterated that the BJP's proposal that chief minister will be from the party which will send more lawmakers to the assembly was not acceptable to the Shiv Sena.
"I want to see a Sena chief minister and I will work for it," he said.
"I have already won in the treaty, now we should win the real battle of election," Mr Thackeray said.