An urea sensor was developed for the determination of urea in dialysate of hemodialysis. The urea sensor was constructed by combining an immobilized enzyme urease membrane with an ammonia electrode. The sensor responded to urea in concentration over 1-100mg/dl. The sensor is useful for the determination of urea clearance of dialyzer. The sensor can also provide a new laborsaving procedure for the estimation of blood urea concentration by measuring urea concentration in outlet dialysate during hemodialysis treatment.