High performance reversed shear (RS) discharges with strong internal transport barrier (ITB) and flat pressure profile in the plasma core region disrupt frequently even with low beta. We analysed MHD instabilities leading to low beta disruption with measuring fluctuations and current profile with MSE measurement. We mainly observed two types of disruptions. One is the disruption without precursor at surface q ∼ integer. The other is the disruption with n = 1 precursor of γ > 100 ms. The poloidal mode number of the n = 1 mode is equal to the outermost integer of q. The n = 1 mode exists continuously from the peripheral region to the ITB layer or the n = 1 modes exist separately at the peripheral region and at the ITB layer. The phase has a difference of 180° between at the peripheral region and at the ITB layer. To explain these characteristics of disruption, we introduce a simple model where disruption occurs when both MHD instabilities at the plasma surface and at the q = m/n surface in the RS region, where m and n are the poloidal and the toroidal mode numbers of the surface MHD instability, are simultaneously unstable. This simple model can explain almost all observed disruption by two processes. One is the surface mode triggered disruption, which occurs when surface q changes; the corresponding q = m/n surface at ITB layer changes discretely. The other is the internal mode triggered disruption, which occurs when the corresponding q = m/n surface becomes unstable gradually.