An　application　of　the　optical　pyrometer　is　studied　for　measuring　monochromatic　emissivities　of　cement　clinker　with　various　Fe20　3　content.　The　idea　of　using　＂brightness　temperature＂　is　introduced　into　the　emissivity　measurement.　In　this　method,　there　is　no　need　for　measuring　an　actual　temperature　of　sample　surfaces,　only　with　determining　both　brightness　temperatures　of　a　sample　and　a　blackbody　can　the　required　emissivity　be　evaluated　according　to　Wien＇s　radiation　law.　In　practice~　the　cement　clinker　is　regarded　as　a　greybody,　the　monochromatic　emissivity　is　approximately　equal　to　the　total　emissivity,　so　a　single-colour　optical　pyrometer　is　applied　for　this　purpose.　Test　measurements　are　carried　out　on　10　kinds　of　cement　clinkers.　Experimental　data　are　treated　by　the　least　square　method.　As　a　result,　the　emissivity　variation　with　temperature　at　a　certain　Fe203　content　is　quite　well　represented　by　e.,　=　a　+　bT.　Furthermore,　this　work　first　reported　that　the　emissivities　of　cement　clinker　change　considerably　with　Fe203　contents.　In　multiple　cement　production　this　conclusion　is　very　important.