As of 7.30pm, Dr Wan Azizah garnered 15,055 votes while Barisan Nasional's Datin Paduka Chew Mei Fun secured 9,628 votes for a majority of 5,427 votes.
Another 3,800 plus votes are still being counted at press time.
Barisan is believed to be able to reduce the 6,824 vote majority obtained by PKR assemblyman Lee Chin Cheh in the May general election last year in the final tally.
The voter turnout for the by-election was 72 percent (28,314 voters) against a turnout of 87.9 percent in the May 5 general election last year.
Kajang has a total 39,728 registered voters in the state seat, which fell vacant following the resignation of its PKR assemblyman Lee Chin Cheh.
In the 2013 general election PKR’s Lee garnered 19,571 votes followed by Lee Ban Seng (BN-MCA) with 12,747 votes and Mohamad Ismail (Berjasa) 1,014 votes.
The three independent candidates who contested the seat are Mohd Iwan Jefrey Abdul Majib (249 votes), Ong Yan Foo (85 votes) and Mohd Khalid Kassim (83 votes).
Lee won the seat with a comfortable 6,824 vote majority