The　fuel　assembly　or　core　with　unstructured　geometry　is　frequently　used　in　the　advanced　reactor.　To　calculate　the　fuel　assembly,　the　transmission　probability　method　(TPM)　is　widely　used.　However,　the　rectangular　or　hexagonal　meshes　are　mainly　used　in　the　TPM　codes　for　the　normal　core　structure.　The　triangle　meshes　are　most　useful　for　expressing　the　complicated　unstructured　geometry.　Even　though　the　finite　element　method　and　Monte-Carlo　method　are　well　suited　for　solving　the　unstructured　geometry　problem,　they　are　very　time-consuming.　Therefore,　a　TPM　code　based　on　the　triangle　meshes　is　developed　here.　This　code　was　applied　to　the　hybrid　fuel　geometry,　and　compared　with　the　results　of　the　MCNP　code　and　other　codes.　The　results　of　the　comparison　were　consistent　with　each　other.　The　TPM　with　triangle　meshes　can　thus　be　applied　to　the　two-dimensional　arbitrary　fuel　assembly.　©　Higher　Education　Press　2007.