Zr-xNb alloys (x = 0.2, 0.8, 1.5 wt.%) were prepared to study the characteristics of the phase transformation in Zr-Nb system. The samples were heat treated at ${eta}$-temperature ($1020^{circ}C$) for 20 min and then cooled with different cooling rate. The microstructures of the specimens having the same compositions were changed with cooling rate and Nb content. The Widmanst$ddot{a}$tten structure was observed on the furnace-cooled sample. The relationship between ${alpha}$-Widmanst$ddot{a}$tten and ${eta}$-phase was the {0001}${alpha}$//{110}${eta}$, <11$ar{2}$0>//<111>. The ${eta}$-phase in Widmanst$ddot{a}$tten structure of Zr-Nb alloys containing Nb more than solubility limit was identified as ${eta}_{Zr}$ phase which was a stable phase at high temperature. In the water quenched samples, two kinds of martensite structures were observed depending on the Nb-concentration. The lath martensite was formed in Zr-0.2, 0.8 wt.% Nb alloys and the plate martensite having twins was formed in Zr-1.5 wt.% Nb alloy.