Methods based on the first-order plus time delay (FOPTD) model are very popular for tuning proportional-integral (PI) controllers. The FOPTD model-based methods are simple and their utility has been proved with many successful applications to a wide range of processes in practice. However, even for some overdamped processes where the FOPTD model seems to be applied successfully, these empirical FOPTD model-based methods can fail to provide stable tuning results. To remove these drawbacks, a PI controller tuning method based on half-order plus time delay (HOPTD) model is proposed. Because FOPTD model-based methods can be applied to higher order processes, the proposed HOPTD model-based method can be applied to higher order processes as well. It does not require any additional process information compared to the FOPTD model-based method and hence can be used for overdamped processes in practice, complementing the traditional FOPTD model-based methods. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 601-609, 2017