In this paper, we investigate the effects of viewing conditions and rotation methods on different types of collaborative tasks in a tabletop AR environment in which two users are co-located. The viewing condition means how the manipulation of a tabletop world by one user is shown in the other users' view and the rotation method means what type of input devices is used to rotate the tabletop world for alternative orientations. Our experiment considered two different viewing conditions - consistent view and inconsistent view and two different rotation methods - direct turn and indirect turn. In the experiment, a 3D display environment called "Stereoscopic Collaboration in Augmented and Projective Environments (SCAPE)" was utilized as a test environment, and two tasks were considered: Lego-like block building task and text label selection task. The former was designed for synchronous and referring-strong type, and the latter was designed for asynchronous and orientation-strong type. As dependent variables, various objective and subjective measurements including task completion time, quality of task result, turn angle, and questionnaire were measured. According to the results, the viewing conditions had significant effects on several objective and subjective measurements. On task completion time, their effect for the synchronous task was opposite to that for the asynchronous task. On the other hand, the rotation methods had significant effects only on turn angle.