Methanol steam reforming is regarded as a very promising process to generate H-2 suitable for fuel cells. Typically, the Pd-based catalysts can catalyze efficiently methanol steam reforming for hydrogen production. But their high selectivity to CO, a byproduct of methanol reforming reaction, severely limits their potential application. In this work, a series of Nb-modified Pd-Zr-Zn catalysts with different Nb loadings were prepared to study their catalytic activities with more focus on the role of Nb on Pd-Zr-Zn catalyst for methanol steam reforming. The prepared catalysts were fully analyzed by using various characterization techniques, for example, ICP, BET, SEM, XRD, H-2-TPR, NH3-TPD, HRTEM, CO chemisorption, XPS, and Raman. The experimental results showed that an increase in Nb loading for the Nb-modified Pd-Zr-Zn catalysts led to a decrease of the methanol conversion and H-2 production rate. This was probably due to the decrease in the amount of oxygen vacancies on the catalyst surface. However, introduction of Nb into Pd-Zr-Zn catalyst increased the acid strength on the catalytic surface. The aldehyde species derived from methanol decomposition were readily transformed to HCOOH, thus yielding high selectivity to CO2 for the Nb-modified Pd-Zr-Zn catalysts. Significantly, the addition of Nb to Pd-Zr-Zn catalyst facilitated the incorporation of Pd into the ZnO lattices, which led to the formation of Pd-Zn alloy. Consequently, the Nb-modified Pd-Zr-Zn catalysts exhibited significantly lower CO selectivity and production rate than the Pd-Zr-Zn catalyst. From the results, this work offers a new way to the rational design of selective methanol steam reforming catalysts to decrease the formation of byproduct CO. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.