Nb-Si based ultrahigh temperature alloys were prepared by vacuum non-consumable arc melting and the effects of Ti addition contents on their microstructure and phase composition were investigated. The results showed that Ti addition contents could exert an obvious influence on the types, sizes and area fractions of primary (Nb,X)5Si3 in the microstructure. When Ti addition content was relatively low (0-10 at.%), the primary silicides were α-(Nb,X)5Si3 and its area fraction increased significantly with increase in Ti addition content. When the Ti addition content was ranged from 18-20 at.%, the primary silicides were of two types: respectively α-(Nb,X)5Si3 and γ-(Nb,X)5Si3. However, the amount of α-(Nb,X)5Si3 decreased with increase in the Ti addition content in the alloy. When Ti addition content in the alloy varied from 22-30 at.%, there was only one form of primary silicides i.e., γ-(Nb,X)5Si3 left. Moreover, as the Ti addition content in the alloy increased, the size of α-(Nb,X)5Si3 increased while the size of γ-(Nb,X)5Si3 was less affected. In addition, as the Ti addition content in the alloy increased, Ti concentration in both Nbss and γ-(Nb,X)5Si3 increased while that in α-(Nb,X)5Si3 was lower and did not change obviously. It should also be noticed that Ti atoms only replaced Nb atoms in various phases while had no obvious effects on the solution tendency of other alloying elements such as Al, Cr and Hf.