The ultrastructure on the dorsal vessel of 5-day-old cabbage butterfly, Pieris rapae L., was carried out using the transmission and scanning electron microscope. The results are as follows. 1) The aorta. The aorta is simple tubular type and consists of the inner and outer membrane of the myocardium and thick myocardium is located between them. However the inner membrane with $0.26{mu}m$ thickness and outer membrane with $0.08{mu}m$ are composed of fibrous materials, the former is composed of low and high densed fibrous materials and the latter appears homogeneous layer. The myocardium consists of typical striated muscles. The sarcomere with $1.6{mu}m$ length and in cross section, each thick filaments are surrounded by $7{sim}8$ thin filaments. The intercalated disc is joining the end of the two muscle cells, desmosomes and septate junctions are appeared between the neighboring muscle cells. 2) The heart. The heart composing of myocardium enclosed by its inner and outer membrane as the aorta has a series of well formed segmental chamber. The arrangement of myofilaments, cell adhensions and membrane elements are observed as same as at the aorta. The inner membrane of the heart is deeply invaginated into the myocardium than the outer membrane and a lot of well developed mitochondria with rod shape are aggregated in the folds. The longitudinally and transversely oriented tubule system formed by invagnation of the sarcolemma into the muscle bundle is built up dyad with the sarcoplasmic reticulum as the aorta. The slit is formed by deeply invagination of the inner membrane of myocadium toward the muscle layer and then the inner and outer membrane of myocardium are fused. Therefore, the ostium is formed between the myocardium and situated at the lateral side of the myocardium.