We examined in this work the dynamics of crack in drying sludge cake using an Xray micro-computerized tomography scanner (micro-CT). The three-dimensional crack structure model was reconstructed, and the geometric parameters of cracks were evaluated over time. Internal cracks were found to grow fast in the initial phase of drying, and to then develop into shapes that exhibited zigzagging with non-circular cross-sections and fractal-like boundaries. The role of cracks on sludge drying is discussed.