The　effects　of　aluminium　addition　on　the　precipitation　behaviour　and　amount　of　pro-eutectoid　carbide　in　ultrahigh　carbon　steels　(UHCs)　were　investigated　by　thermodynamics　calculation　and　SEM　microstructure　analysis.　It　is　shown　that　there　exists　a　temperature　range　(A1b-A1e)　rather　than　a　definite　eutectic　transformation　temperature　for　eutectoid　transformation　in　Fe-C　binary　alloy.　Furthermore,　both　the　upper　limit　(A1e)　and　the　lower　limit　(A1b)　of　this　temperature　range　increase　with　aluminium　content　in　UHCs.　The　precipitation　of　pro-eutectoid　carbide　was　inhibited　remarkably　in　UHCs　containing　less　than　1.6%　wt　carbon　by　the　addition　of　more　than　2%wt　Al.　The　amount　of　pro-eutectoid　carbide　in　UHCs　decreases　with　the　addition　of　aluminium,　especially　with　greater　than　2%wt　Al.　The　lamellar　spacing　of　perlite　in　UHCs　containing　aluminium　is　smaller　than　that　in　UHCs　free　of　aluminium.