The Italian astronomer Galileo Galilei discovered the isochronism of the pendulum by accident around 1581. He measured the swinging motion of a chandelier in the Pisa cathedral by his pulse. He found the swinging period not varied even the weak swinging amplitude. In weak swinging, the period of the pendulum motion is only relative to the length of the pendulum. In this paper, I will show how to do a pendulum experiment with smartphone. Students can learn the physics of the pendulum by do-it-youself experiment anytime and anywhere.