This paper can be divided into four main parts in which following numbers are explained: 1. What is roundness and why it is important to measure out of roundness; 2. Procedure of out of roundness measurement; 3. Present stylus calibration method of roundness measuring instrument; 4. New stylus calibration method of roundness measuring instrument. In the first part, term „roundness“ is defined. Also, importance of out of roundness measurement is explained. Several typical forms of roundness errors are listed, as well as influence of roundness on funcionality of an object. In the second part, two types of roundness measuring instruments and numerical methods that are used for roundness determination are explained. Also, the procedure for roundness measurement in the Laboratory for Precise Measurement of Length (LFSB) of the Faculty of Mechanical Engineering and Naval Architecture is demonstrated, step by step. The third part refers to the present calibration method of the roundness measuring instrument Mahr Parthen MMQ-3. Standards used for calibration of the roundness measuring instrument, as well as the procedure of static stylus calibration, using universal length machine (ULM) Joint Microrep DMS 680, are illustrated. The procedure is presented in the way it has been used in LFSB. The last part contains results of the present static stylus calibration method and results of new static stylus calibration method using piezo actuator. Suitability of application of this new stylus calibration method is examined by comparing results of previously mentioned methods. Additionally, dynamic stylus calibration is carried out using piezo actuator for Flick standard simulation.