MIBS is a lightweight block cipher designed by M. Izadi et al in 2009 for tiny computing devices, such as RFID tags and sensor network nodes. The cipher algorithm iterates a Feistel structure with SP type round function by 32 rounds. It with 64-bit block size and supports 64/80-bit key size. In this paper, a new related-key impossible differential attacks to reduced-round MIBS with 80 bit keys is presented. By carefully choosing the relations of keys, a new 8 rounds related-key differential trials is constructed, by using which we first break 15-round MIBS with 80 bit keys. This result improves the known impossible differential attack on MIBS-80 by one round.