Antiphospholipid syndrome (APS) is an autoimmune systemic disorder characterized by arterial, venous, or small vessel thrombosis, and/or recurrent early pregnancy loss, fetal loss, or pregnancy morbidity.APS is induced by persistent positive antiphospholipid antibodies (aPL), the main being lupus anticoagulant (LA), and/or anticardiolipin (aCL) antibodies, and/or anti-beta 2 glycoprotein 1 (β2-GP1) antibodies. Some studies have shown that the incidence of APS is about 5 new cases per 100,000 persons per year, and the prevalence is around 40-50 cases per 100,000 persons. APS can be primary or secondary. Secondary APS often coexists with another autoimmune disorder, most commonly systemic lupus erythematosus (SLE).Behcet's disease (BD) is usually characterized by recurrent oral and genital aphthous ulcers and ocular involvement. It can occasionally affect the venous system. BD usually affects small vessels, but can sometimes affect large veins or even a variety of veins. Because most of APS is secondary to SLE, APS secondary to incomplete BD is quite rare. This report describes a case in which a 15-year-old male experienced bilateral leg swelling and pain. The patient had a long history of self-healing recurrent mouth ulcers. Laboratory tests revealed positive β2-GP1 immunoglobulin A (IgA). His symptoms improved by using steroids, prednisolone, uro-kinase, and hirudin. In this rare case of secondary APS, the patient was diagnosed with anti-β2-GP-1 IgA positive to incomplete BD. It is a rare case of secondary APS with positive anti-β2-GP1 IgA to incomplete BD. It is suggested that patients with recurrent mouth ulcers should be closely examined to prevent thrombosis, and more laboratory markers should be used to avoid a risk of misdiagnosing patients with APS.