Aggression is an innate social behavior and aggression directed toward a stronger opponent in animals can lead to severe physical damage or even death. Therefore, disadvantageous fights to survive and reproduce need to be avoided. This study identified the projection from estrogen receptor alpha-expressing cells in the caudal part of the medial preoptic area (cMPOA^Esr1) to the ventrolateral part of the ventromedial hypothalamus as an essential pathway for modulating aggression in male mice. They performed fiber photometry recording of MPOA^Esr1 calcium signal to examine the potential heterogeneity in MPOA^Esr1 cell responses during social behaviors. This dataset includes fiber photometry recording data, behavior annotations, and raw representative histology images. The data reveal that cMPOA^Esr1 is a key population for encoding opponents’ fighting capability.