Cost efficiency plays a significant role in bank risk taking behaviour. This paper examines the effect of cost efficiency on the liquidity risk of Islamic banks and conventional banks in 16 OIC countries from 1999 to 2013. The findings suggest that cost efficiency has a positive effect on liquidity risk. Other significant factors of liquidity risk include capital, bank specialization, credit risk, profitability, size, GDP and inflation whereas market concentration is not significant contributor to banking liquidity risk. There is weak evidence to support the notion that Islamic banks have higher level of liquidity risk than conventional banks. The findings imply the need to provide liquidity, probably through a well-functioning money market to lower liquidity risk in banking.