The crack growing behavior in a brittle rectangular plate under transient thermal stress is studied. The transient thermal stress imposed to the rectangle here is that of due to a line heater such as an electric resistance wire which supposed to be perfectly contact to the surface of material. When the heater is placed on the symmetrical axis of the rectangle, the crack propagates just along the heating line, therefore, this situation can be used for dividing a thin rectangular plate into equal two pieces. Contrast to the symmetrical case, on the other hand, the warping of crack propagation path is observed when the heater is placed other than the symmetrical position. The main objective of the present study is to clarify the mechanism of crack path warping in the case of unsymmetrical cleaving through the crack propagation simulation. The simulated and the observed crack path show good agreement with each other and factors which make the crack path warp are understood
