They comprise 15 Buddhist monks and a Christian, Hindu and Islamic priest each.
They are found guilty of murder, rape, robbery, rioting and conspiring against the state.
The Hindu priest was sentenced over an attempt on the life of former president Chandrika Kumaratunga.
The CHR has found that all 18 of them are serving time as per provisions of the prisons ordinance which deals with their food, clothing, visitors and each given a number.
If members of the clergy sent to prison need to be given special treatment, parliament has the powers to formulate new laws, it says.