A phosphate ion-selective electrode (ISE) based on molybdenum was developed, which is based on the reaction between molybdenum and hydrogen phosphate ions (HPO42−) in alkaline condition. We found that the electrode showed a near Nernst characteristics with a slope of −26.9 ± 0.5 mV/decade. Potential linear range is from 10−1 M to 10−5 M and detection limit is 1.9 × 10−6 M in the pH of 8.5. The short response time for HPO42− ions indicates its fast response characteristic. The ISE can be used over a period of three months with good reproducibility and selectivity. The manufacture process of the ISE is easy and inexpensive. It can be applied to quantitative analysis of phosphate in water.