The crack propagation in austenite phase of the ferrite-austenite duplexstainless steel, observed in situ under TEM, was found as zigzag path frequently.The main crack and the microcrack, nucleated in the austenite grain ahead of themain crack, caused an interaction. The choice of the secondary slip system at micro-crack tip and the propagating path of microcrack are controlled by the microcracktip stress field and the stress field of the main crack.