The use of finger on the touchscreen is one of the most prevalent forms of input on mobile devices. However, due to the size of the finger tip, precise input is difficult and the presence of the finger on the touchscreen can often occlude the content in interaction. In this paper, we propose to install a sensor in the corner of the mobile device (e.g. smart phone) and transform the mobile device into a digital pen for making input for itself or other external devices. The system, dubbed "CornerPen" has many potential advantages in addition to those of the traditional pen-based input (vs. finger based) such as less occlusion, leveraging on tactile memory, and larger interaction surface. We have implemented and experimentally tested the CornerPen against the nominal finger-based touchscreen input system using two tasks, namely, flick-based icon browsing (search) and selection and free-form text input. Our results showed while the subjects did acknowledge the problem of occlusion with finger-based input on the touchscreen, the CornerPen approach still was not particularly effective nor preferred for the intended purpose, i.e. making precise input, and only exhibited comparable performance for simple flick/tab like input actions.