We used a new rapid data acquisition system to examine the role of pre‐existing crack density in determining the mechanical properties of crystalline rock. Two similar coarse‐grained samples, Tsukuba granite (TG) and granitic porphyry (GP), were used in triaxial compression tests. The main difference between the two rocks is that TG contains main large pre‐existing cracks, whereas GP is almost crack‐free. In our tests, the two rocks showed significantly different behaviors. In GP, before the fault nucleation, AE activity was low and showed increasing b‐values with increasing stress. But in TG, a large number of AEs were observed and showed short‐term b‐value anomalies ‐ mutual fluctuations on a decreasing background. The short‐term fluctuations of b‐values are closely correlated with those of the spatio‐temporal clustering of AE locations. Our results demonstrate: 1) pre‐existing cracks are the most dominant factor of all heterogeneities that govern the fault nucleation process in laboratory rock samples; 2) failure predictability is dependent on the pre‐existing crack density.