Matter of proof in civil cases is important since the parties have to prove what is being disputed. It is stated in Article 163 HIR that whoever owned the rights to something, he has to prove it. There are some evidences which have perfect strength of proof, namely, the written evidence, witness, suspicion, recognition, and the oath. But in practice, the problem arises. Letter of Interrogation of the Suspect was used as an evidence by the judge in the decision number 06/Pdt/2001/PN.Ciamis. As analyzed in this paper, the LOI is only a preliminary evidence so other evidences are needed to make it enforceable as a perfect proof, as stated in Article 164 HIR in conjunction to Article 165 HIR.