The nature of the mental representation during the mental rotation process is controversial. In the present experiment, this issue was investigated by manipulating the integrity (reference lines) and complexity (square numbers) of the stimuli. The monotonic relation between transformation time and angular difference was found and interacted with the integrity of the stimuli. As reference lines in a picture specified relative spatial relation among the picture's components which have clear space relation, participants did not rely on the ability of mental rotation to achieve this task. However, there were individual differences in the perception of spatial relations in the pictures. Nevertheless, stimulus complexity increased the overall reaction time but did not affect the linear relationship. We inferred there are two subtle processes in the mental process of mental rotation-rotation of a reference line and reconstruction of the image. The degree to which participants relied on each process was modulated by the characteristics of the stimuli.