Morphological aspects of thermally induced brittle cracks are revisited. The formation of a growth pattern of cracks, which are subjected to the thermal loading due to the penetration of surface cooling, had been extensively discussed for the system of parallel interacting straight cracks. In the present paper, attentions are focused on the transition from straight to wavy crack propagation of edge cracks due to the penetration of surface cooling in a brittle solid. Curved crack extension is predicted based on the local symmetry criterion, where step-by-step finite element calculations are carried out by the automatic mesh generation using the paving method. Based on the variety of numerical results, the influencing factors which control the wavy crack paths are identified. Qualitative discussions are also made by comparing the numerical results with some experimental observation.