I‐detectability is an important issue in partially observed discrete event systems (DESs). In this paper, a Boolean semi‐tensor product (BSTP) approach is proposed to investigate the I‐detectability problem of partially observed DESs. First, two concepts of I‐detectability, that is, strong I‐detectability and weak I‐detectability, are recalled for partially observed DESs. Second, in order to estimate the initial state of DESs after finite observation events, an observer is constructed and converted to an algebraic form based on BSTP. Third, based on the algebraic form of observer, two necessary and sufficient conditions are presented for strong I‐detectability and weak I‐detectability of partially observed DESs, respectively. Finally, two examples are given to illustrate the obtained results.