This research constructs a relationship between the geometrical cross section and the fatigue strength of aluminum alloy (6063). Different geometrical cross - section specimens were manufactured with the same section space, the specimens were tested under bending and unbending loading conditions. The strain was calculated and fatigue curves were drawn for all different forms of sections. After studying and analyzing the results, it became clear that increasing the sides in any form increases the fatigue strength as the strain is fixed. Fatigue curve equations were derived using computing techniques and connected to find a mathematical relationship between the geometrical shape of the section and fatigue strength. Equation were derived in order to determine the stress or strain, a specimen of a certain section figure can endure to fail under the effect of a number of cycles. The results show a good agreement when applying the same conditions for all specimens.