We present GyroPen, a method for text entry into mobile devices using pen-like writing interaction reconstructed from standard built-in sensors. The key idea is to reconstruct a representation of the trajectory of the phone's corner that is touching a writing surface from the measurements obtained from the phone's gyroscopes and accelerometers. We propose to directly use the angular trajectory for this reconstruction, which removes the necessity for accurate absolute 3D position estimation, a task that can be difficult using low-cost accelerometers. Recognition is then performed using an off-the-shelf handwriting recognition system, allowing easy extension to new languages and scripts. In a small user study (n=10), the average novice participant was able to write the first word only 37 seconds after the starting to use GyroPen for the first time. With some experience, users were able to write at the speed of 3-4s for one English word and with a character error rate of 18%.