High cell density cultivation of Haematococcus pluvialis for astaxanthin production was carried out in batch and fed-batch modes in 3.7-L bioreactors with stepwise increased light intensity control mode. A high cell density of 2.65 g x L(-1) (batch culture) or 2.74 g x L(-1) (fed-batch culture) was obtained, and total astaxanthin production in the fed-batch culture (64.36 mg x L(-1)) was about 20.5% higher than in the batch culture (53.43 mg x L(-1)). An unstructured kinetic model to describe the microalga culture system including cell growth, astaxanthin formation, as well as sodium acetate consumption was proposed. Good agreement was found between the model predictions and experimental data. The models demonstrated that the optimal light intensity for mixotrophic growth of H. pluvialis in batch or fed-batch cultures in a 3.7-L bioreactor was 90-360 micromol x m(-2) x s(-1), and that the stepwise increased light intensity mode could be replaced by a constant light intensity mode.