The shock-induced structural instability for three kinds of maraging steel is studied with two experimental techniques: Taylor technique and uniaxial strain conditions. Spall strength of steels is shown to be determined by the structural instability threshold under compression at the front of compressive pulse. Microstructure investigation of post-shocked specimens show that structural instability of maraging steel results from two factors: (i) disintegration of strengthen phase created in the process of aging of steel and (ii) shock-induced nucleation of localized austenite inclusions responsible for decrease of yield stress and fragmentation. Dynamic fracture of maraging steels is found to occur along the boundaries between shock-induced austenite laths and rest of the matrix.