Recent introduction of low-osmolar nonionic contrast agents has allowed the performance fo angiography with certain advantages such as reduced pain, reduced osmotic load and other potential advantage, over high osmolar ionic contrast agents. But the potential thrombogenic risk of nonionic contrast agent has been debate because of their weak anticoagulation effects. Several reports have recently documented the formation of thrombi in catheters and syringes containing nonionic contrast agent, and thromboembolic episodes have been noted during angiographic procedures. We have also been experienced blood clotting within blood mixed contrast agent syringe during angiography. Thus, we have studied with blood mixed ionic(Diatrizoate, Ioglicate) agents and nonionic(Iopamidol, Iopromide) agents, that used usually in our hospital, and saline in plastic and glass syringes. Each syringes were checked the clot formation on 10,30,60,90 minutes. Total 340 samples were obtained from 8 adults before angiography. Our data showed that nonionic contrast agents had signficantly lesser anticoagulation effect than ionic contrast agents(P<0.0001 on Chi-square test), both in plastic and glass syringes. And formation of clotting in glass syringes were signficantly greater than that in plastic syringes(P<0.0001). Thus meticulous technique is required to prevent thrombosis during angiographic procedure using nonionic contrast agents.