Silica-alumina catalysts modified with different loadings of phosphorus have been used for the skeletal isomerization of 1-butene to investigate the effects of the phosphorus modification on the conversion of 1-butane and the selectivity for iso-butene. Increasing the Phosphorus loading led to a decrease in conversion of 1-butene over phosphorus-modified silica-alumina catalysts, while the selectivity far iso-butene increased. Silica-alumina catalyst with phosphorus of 9.8 wt% showed an extremely high selectivity for iso-butene. The results of NH₃-TPD, adsorption-desorption behavior of 1-butane and TG analysis of used catalysts indicate that the considerable decrease of strong acid sites by phosphorus modification causes the high selectivity for skeletal isomerization of 1-butene.