This paper uses a three-axis accelerometer to design and implement a three-dimensional pedometer, which can determine the walking or jogging automatically. This device can count the number of steps. It can also judge the status of walking or jogging simultaneously. The three-dimensional pedometer can be worn on the user's waist or placed within pocket arbitrarily. This three-dimensional pedometer can also detect the intermittent steps. It also improves the general pedometers' judgment mechanism which the users have to exercise continuously. Some different smoothing methods is used to filter the noises of human walking reacting force. In the determination of walking and running, this paper presents the filter by walking threshold and running threshold, and designs an algorithm to switch walking mode and running mode.