Interacting with digital contents in 3D is an essential component to various applications (e.g., modeling packages, gaming, virtual reality, etc.). A traditional interface such as keyboard-mouse or trackball usually demands non-trivial working space as well as a learning process. We present the design of EZ-Manipulator, a new 3D manipulation interface on smartphones that supports mobile, fast, and ambiguity-free interactions with 3D objects. Our system leverages the built-in multitouch input and gyroscope sensor of a smartphone to achieve 9DOF (nine Degrees of Freedom) axis-constrained manipulations and free-form rotation. Thus, using EZ-Manipulator to manipulate objects in 3D is easy. The user merely has to perform intuitive single- or two-finger gestures and rotating the device in hand(s) to achieve manipulations at respectively fine-grained and course level. We further investigate the ambiguous manipulations introduced by the indirect manipulations using multitouch interface and propose a dynamic virtual camera adjustment to effectively resolve the ambiguity. A preliminary study reports that our system has significant lower task completion times in comparison to the conventional keyboard-mouse interface, and receives positive user experience from both novices and experts.