A remarkably high thermoelectric figure of merit ZT of ~2.6 at 923 K was recently reported in p-type SnSe single crystals along the crystallographic b-axis. Afterward, realizing comparable thermoelectric performance in the polycrystalline counterparts have been intensively investigated. However, polycrystalline SnSe-based materials unexpectedly exhibit much higher thermal conductivity than the single crystals, resulting in far poorer thermoelectric performance in the former. In this presentation, we hypothesize that surface oxidation would be the main origin of their high thermal conductivity. To solve this issue, we introduce an oxygen-removal process under mildly reductive atmosphere. As a result, we are able to achieve the ultralow lattice thermal conductivity and thereby markedly enhanced ZT in polycrystalline SnSe-based materials, which are comparable to those in their single crystals.