Diamond has attractive properties as an advanced electronic material. Its combination of high carrier mobility, electric breakdown, and thermal conductivity results in the largest calculated figures of merit for speed and power of any material. Recently, the discovery and development of what might be called sophisticated secondary effects in diamond, are opening interesting sensor applications. Boron doped diamond will change electrical resistance with strain (piezoresistance), meaning it can be used as a strain gauge on rugged electronic microsensors for pressure and acceleration sensing. This paper will present some critical issues of diamond for microelectromechanical sensing applications such as its rupture stress and edge stress of diamond diaphragms and the high temperature responses of a diamond pressure sensor. We will describe an all diamond pressure microsensor that measures pressure at more than 300°C. Also, we have observed that layered diamond films can behave as chemical sensors measuring hydrogen, oxygen and many other chemicals’ concentration. For example, a diamond-based chemical gas sensor using Pd/i-diamond/p+-diamond metal-insulator-semiconductor diode structure has been made and the behavior of a new diamond-based chemical gas sensor has been studied. Also, the creation and properties of diamond microtips as field emitters are discussed.