This work was carried out aiming at studying at the light microscope the diagonal artery of the pig's heart to analyse the composition of the arterial wall and the morphometry of its tunicas as well. The hearts of twelve pigs of both sexs were used. From those 12 hearts, six had diagonal artery and other six tacked it. From those with the artery 18 blocks of 1cm² were removed from the intimal segment of the arteries, six blocks bling form the diagonal artery, six from the anterior interventricular arteries and six from the circunflex arteries. From another 6 hearts without diagonal artery 12 blocks of the 1 cm² were removed, six blocks bling from the anterior interventricular arteries and six from the circunflex arteries. In each arterial segment 20µm thick histological sections were made. The laminal obtained were stained by the following methods: resorcine-fucsine of Weigert counter-stained with picrofucsine of van Gieson; method of Verhoef; method of Azan . It was observed that the diagonal arteries belonged to the groups of muscular, or medium diameter arteries. As for the histomorphometric analysis, considering the tickness of the wall of the anterior interventricular, circunflex and diagonal arteries, it was observed that a larger thickness occured at the epicardial wall, followed by the miocardial wall. It was demonstrated a larger wall in the anterior interventricular artery, followed by the circunflex and then the diagonal arteries. Comparing the mean thickness of the wall of the anterior interventricular artery in the hearts with or without diagonal artery, it was observed that in the former it was thinner than in the latter. This finding was also made for the mean thickness of the wall of the circunflex artery in the hearts with ou without diagonal artery, showing that presence of the diagonal artery determns a change in the initial thicness of the walls of the two arteries