Conductive boron-doped diamond (BDD) electrodes are very attractive electrode materials because of their superior electrochemical properties such as wide potential window, low background current, chemical inertness, and mechanical durability. We have studied the application of diamond electrodes as electrochemical sensors for the detection of several types of biochemical species or environmental pollutants. For example, free chlorine, metals (Zn, etc), and proteins including cancer markers could be detected by using the diamond electrodes with very high sensitivity and high stability. Furthermore, modified diamond electrodes such as ion-implanted diamond electrodes and diamond electrodes with micro-meter size were also prepared in order to improve the electrochemical properties. Here, several examples for electrochemical applications by using the boron-doped diamond electrodes were introduced.