She was 81.
A Missionaries of Charity official said Sister Nirmala was unwell for the past few months and her health condition had deteriorated since May.
"She was having heart-related ailments since months and her condition deteriorated since May," Archbishop of Calcutta Father Thomas D'Souza told PTI.
The Archbishop visited St John's Church this evening where Sister Nirmala's body has been kept for people to pay their last respect.
He organised a mass prayer and paid his last homage to Sister Nirmala.
West Bengal Chief Minister Mamata Banerjee also paid a visit to St John's Church and condoled Sister Nirmala's death.
"I express my condolence on the demise of Sister Nirmala formerly Superior General of the Missionaries of Charity. This revered nun dedicated her entire life to the services of the distressed. She shouldered great responsibility of Superior General for 12 long years. Her death is (an) irreparable loss. May her soul rest in peace," Banerjee told reporters at Nabanna.
Her body would be kept there till 5 PM to allow people to have a last glimpse. Then her body would be taken to the century-old graveyard of St John's Church for the last rites, the official added.
Sister Mary Prema succeeded Sister Nirmala in April 2009.
