What is a liquid fund. Liquid fund is a category of mutual fund which invests primarily in money market instruments like certificate of deposits, treasury bills, commercial papers and term deposits.Lower maturity period of these underlying assets helps a fund manager in meeting the redemption demand from investors.ow to choose a liquid fund. The returns from liquid funds don't vary much as they invest in similar underlying securities. However, when looking for a liquid fund, the past return should not be the only factor for consideration.