Rani, representing Uttar Pradesh, threw the spear to a distance of 61.86m to better her own earlier national mark of 60.01m in the last event of the competition at the NIS Patiala.
In the process, she also qualified for the World Championships to be held in London in August. The qualifying mark for the World Championships stands at 61.40m.
Suman Devi, also of Uttar Pradesh, took the silver with a best throw of 55.03m, while K Rashmi of Karnataka won the bronze with 51.12m.
The World Championships qualifying mark for women's 400m stands at 52.10 seconds.
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Seasoned M R Poovama of Karnataka came a distant second in 52.70 seconds while Debashree Mazumdar of West Bengal claimed the bronze in 53.59 seconds.
"I was expecting to make a national record here but I am satisfied with the meet record. I will target the national record at the Asian Championships," said Nirmala.
Meanwhile, four athletes took home their second gold of the meet. G Lakshmanan and his sister L Suriya added the 10000m crowns to the 5000m titles they had won on the first day of the competition.
Suriya bagged the women's 10000m crown with a time of 33.12.67. She finished ahead of Maharashtra's Sanjivani Jadhav (33.15.77) while Delhi's Meenu finished third in 34:17.11.
They were joined by Nayana James of Andhra Pradesh, who claimed the women's 100m hurdles gold in 13.96 seconds, to go along with her long jump title earlier in the competition.
Among the double gold winners was Amoj Jacob, only 19, who completed both 400m and 800m and won the men's single lap race today.