Abstract: The Error Correcting Code (ECC) is one of the most used hardening methods in Flash memory. In this study, the effect of ECC failure modes on memory error cross section has been investigated. The errors are counted in the situations of ECC function enabled and disabled separately after heavy ion irradiation. The results show that the upset cross section is a constant value in different ion fluences when the ECC function is disabled. With the ECC function enabled, the error cross section increases with the increase of ion fluences because the ECC failure modes lead to the increase in the number of errors. Moreover, all ECC failure modes are simulated, and their probability is calculated separately. The results help to understand the impact of ECC failure modes on Flash memory’s error cross section induced by irradiation. Some possible mitigation approaches are provided in response to this phenomenon.