Fundamental body characteristics of Peruvian jack mackerel and approximation of girth were studied to determine the mesh size of nets in the future. On specimens collected off the coast of Callao-Peru, the girth, height and breadth at both maximum position (suffix “m”) and operculum position (suffix “o”), weight and fork length were measured. The relationships of girths (G_m, G_o), heights (H_m, H_o), breadths (B_m, B_o) and weight (W) for fork length (L) were presented as G_m/L=0.511, H_m/L=0.188, B_m/L=0.125, G_o/L=0.483, H_o/L=0.169, B_o/L=0.118 and W/L³=0.113×10^-4. Assuming the girth as a circumference of an ellipse, approximation of girths (G_m, G_o) was attempted using maximum height (H_m) and maximum breadth (B_m). The approximation obtained could be used with high precision. The body characteristics of Peruvian jack mackerel is nearly similar to that of large-size Japanese jack mackerel.