Phytohormones function as signal molecules that regulate physiological processes to protect plants from environmental stresses, including aphids’ attack. We studied the pattern within the defense mechanisms of soybean [Glycine max (L.) Merr. cv. “Nam Dan”] regarding the signaling pathways of salicylic acid (SA), and jasmonic acid (JA) in response to cowpea aphid (Aphis craccivora Koch). With infestation by cowpea aphid, SA was the first to accumulate and reached high levels 24 hours post-infestation (hpi). An accumulation of SA in the early response of soybean probably triggers inducible specific defense reactions. Following SA, JA was later induced and continuously increased to high levels 96 hpi. An accumulation of JA in the later response may be a critical step in the signaling of the downstream defense cascade. In addition, phenylalanine ammonia-lyase (PAL, EC 4.3.1.24) and benzoic acid 2-hydroylase (BA2H), enzymes involved in the biosynthesis of SA, and lipoxygenase (LOX, EC 1.13.11.12), an important enzyme in the JA biosynthesis pathway, were also induced by cowpea aphid. The changes in the enzymatic activity of PAL, BA2H and LOX, and expression of gene encoding PAL were closely associated with the accumulation of endogenous SA and JA, respectively. The variations in the levels of these defense-related compounds were strongly connected with density and the duration of cowpea aphid infestation. Different accumulations of SA- and JA-signaling pathways may contribute to a coordinated regulation leading to the formation of resistant lines in the defense mechanisms of G. max cv. “Nam Dan” against A. craccivora.