The phase compositions,microstructure and especially phase interfaces in the as-cast and heat-treated Nb-Ti-Si based ultrahigh temperature alloys have been investigated.It is shown that β(Nb,X)5Si3 and γ(Nb,X)5Si3 are the primary phases in the Nb-22Ti-16Si-5Cr-5AI (S1) (at％) and Nb-20Ti-16Si-6Cr-4Al-5Hf-2B-0.06Y (S2) (at％) alloys,respectively.The Nb solid solution (Nbss) is the primary phase in Nb-22Ti-14Si-5Hf-3Al-1.5B-0.06Y (S3) (at％) alloy.An orientation relationship between Nbss and γ(Nb,X)5Si3 was determined to be (1 (1)0)Nb//(10(1)0)7 and [111]Nb//[0001]γ in the as-cast S2 and S3 alloys.Some originalβ(Nb,X)5Si3 transformed into α(Nb,X)5Si3 because Al and Cr diffused from the β(Nb,X)5Si3 to Nbss during heat treatment at 1500 C for 50 h in the S 1 alloy.Meanwhile,Ti diffused from Nbss to β(Nb,X)5Si3,which induced αTi to generate near the interface between Nbss and Ti-rich β(Nb,X)5Si3.The orientation relationship between the newly-formed αTi and previous Nbss was (110)Nd/(1 （1）0（1）)αTi and [001]Nb//(12（3）（1）)αTi.Among the (Nb,X)5Si3 phases,the contents of Cr and Al in β(Nb,X)5Si3 are neary the same as those in y(Nb,X)5Si3 but obviously higher than those in the α (Nb,X)5Si3,whereas the content of Si in α(Nb,X)5Si3 is nearly the same as that in γ(Nb,X)5Si3 but higher than that in the β(Nb,X)5Si3.