The crack cleavage extension on nanoscale within the triangular lattice of a fcc materials is addressed. The crack propagation behavior of the sample without and with different pre-straining level under tensile strain is simulated using the advanced phase field crystal (PFC) method. It can be founded that for samples with pre-straining along [1¯10] of X direction, the crack propagation characteristics of the extending-turning–extending are presented, which appears the zigzag edge of the crack; while for samples with pre-straining along [1¯1¯2] of Y direction, the crack propagation at the later stage presents longer cleavage crack extension along with the stress concentration of the dislocation at crack tip, which direction is about [12¯1] and [1¯21¯]. The PFC model combined with the diatomic spring model is used to reveal the mechanism of the cleavage crack start-up and extension on nano-scale.