[en] Mixtures of allyl diglycol carbonate (CR-39) and diallyl phthalate (DAP) were cast into resin plates under same polymerizing conditions. The copolymer plates were irradiated with many kinds of ions at the medium energy beam irradiation room or biological irradiation room of National Institute of Radiological Sciences (NIRS)-Heavy Ion Medical Accelerator in Chiba (HIMAC). After etching in an aqueous solution of KOH, whether enlarged ion tracks exist or not was determined by using an optical microscope. It was found that every plate has a characteristic value in LET as threshold for track formation. The threshold increases with the concentration of DAP. It has been shown that the threshold is adjustable by changing the ratio of CR-39 and DAP. It is expected that the copolymers can be used for the discrimination of heavy ions in cosmic rays