The potassium lithium niobate (KLN) crystal is attractive due to its excellent second harmonic generation (SHG) properties. It is difficult to get large sized samples because of cracking. In, this paper, the cracking mechanism of the KLN crystal in relation to its structure is based on the cracking morphology, the thermal mechanical analysis, and crystallochemistry, It is found that the cracking always occurs parallel to the cleavage plane (0 0 1) and that the surface cracking lines are perpendicular to the [0 0 1] direction. The cracking is due to the thermal stress generated by the big difference in the thermal expansion coefficients along different directions during the cooling process after the crystal growth. From 600 degreesC to 20 degreesC, the KLN crystal exhibits a linear expansion along the [1 1 0] direction, but a large contraction along the [0 0 1] direction. The contraction along the [0 0 1] direction increases with decreasing content of Nb. The formation mechanism of the observed cracking is also discussed by the crystallochemistry approach in which the generation of cracking is related to the anisotropy of the inter-atom bond strength.