A structure of an ultrathin clock comprises generally a roughly rectangular housing having partitioned into a first chamber on upper portion for disposing a clockwork mechanism and a dial, a second chamber on median portion for disposing a set of electric circuit and batteries, and a third chamber on the lower portion for pivoting a pendulum therein and a cover which is snap fitted to the third chamber for securing the pendulum. The pendulum has a magnet on the upper end directly under an electric coil inside the second chamber so as to actuate the pendulum to swing laterally upon the variations of the electromagnetic field between the coil and the magnet. This invention has been characterized in the thickness of the housing which is greatly thiner than that of the prior art.