The effects of ultrasonic treatment on fractal microstructures of wet microalgal cells were investigated for lipid extraction. Fractal dimension of cells with distorted surfaces increased with power and ultrasonication time. Microalgal cells shrank owing to dehydration and cytomembranes were reduced to debris, but cell walls were not fragmented. When ultrasonication power increased from 0 to 500 W for 30 min, the fractal dimension of cells increased from 1.21 to 1.51, cell sizes decreased from 2.78 to 1.68 mu m and cell wall thickness decreased from 0.08 to 0.05 mu m. When ultrasonication time increased from 5 to 30 min with a power of 150 W, the fractal dimension of cells increased from 1.24 to 1.37, cell sizes decreased from 2.72 to 2.38 mu m and cell wall thickness first increased to a peak of 0.22 mu m and then decreased. Long-chain and unsaturated lipids were degraded into short-chain and saturated lipids with ultrasonic cavitation. (C) 2014 Elsevier Ltd. All rights reserved.