Abstract: From March to May in 1999, by method of transmission electron microscopy (TEM), effect of cadmium on ultrastructure of myocardial cell of freshwater crab Sinopotamon yangtsekiense Bott (1967) was revealed for the first time. The results showed that the main alterations were both mitochondria and nucleus from 30 minutes to 24 hours. First of all, nuclear membrane appeared swelling. Later, nuclear membrane disintegrated were. The mitochondrial cristae partly disintegrated, fully disintegrated. In the course of time, mitochondial showed swelling and a lot of vacuolus. Besides, the numbers and types of lysosome were increased in the mean time. Myofibril broke and losed contracted function of myocardium.