Along with Chandy, six floor leaders of the UDF coalition were administered the oath of office and secrecy by Governor R S Gavai at the lawns of the Raj Bhavan.
The UDF unseated CPI(M)-led LDF by securing a thin majority of 72 seats in the 140-member House after April 13 Assembly polls.
Those sworn in as ministers were P K Kunhalikutty (IUML), K M Mani (Kerala Congress-M), K P Mohanan (SJD), T M Jacob (Kerala Congress-J), K B Ganesh Kumar (Kerala Congress-B) and Shibu Baby John (RSP-B).
The ministry will be expanded on May 23 with the induction nine Congress nominees, three representatives of IUML and one more from Kerala Congress (Mani).
Chandy`s predecessor and CPI (M) veteran V S Achuthanandan, KPCC president Ramesh Chennithala, Union ministers Mullapally Ramachandran and K C Venugopal were among those who attended the ceremony.
A large number of people from different parts of the state turned up to witness the swearing-in.
Chandy is becoming Chief Minister for the second time after an interim term in 2004-06.
The UDF had yesterday wrapped up an agreement with its partners on sharing the ministerial berths and portfolios.