Heng Savy the accuser of Cheam Channy, opposition Sam Rainsy party's Member of Parliament, committed suicide in April in Pursat Province.
The news of Heng Savy's death just emerged a few weeks before Cheam Channy's trial starts in the military court.
Cheam Channy is accused of forming illegal armed forces against Prime Minister Hun Sen.
Ker Thy, Cheam's lawyer, said that the death prevents him from asking the plaintiff at the court hearing if he acted alone or if he had received orders from someone else.
SRP's Secretary General Eng Chhay Eang reacted by saying the suicide should be investigated.
A Human Rights activist in Pursat Province investigated the death and told VOA that Heng Savy hung himself near a police post, but gave no further details.
Provincial police chief and military court investigating judge, Pok Porn, confirmed the death.