The present paper shows the design a development and on-line measure system for blood urea concentration during dialysis treatment, using non-invasive technical by employ the optical sensing in the visible range. Previous results obtained in Military Hospital Centre in Mexico DF., displayed experimental evidence of a clear correlation between blood urea contents and optical absorbance in the red region obtained through the arterial line in hemodialysis machine. Our hypothesis considers that the cause of this dependence is the volume change of the red cells with the urea concentration due to osmotic pressure and regulation mechanism in cellular volume. The reduction in the cells volume would lead to decrease in the proportional absorbance value. Measurements were performed during hemodialysis procedures in the Military Hospital Center in Mexico City, show good correlation. The most relevant interfering parameter was the water extraction along the dialysis treatment, given that changes in the blood fluid and arterial pressure (hypertension, hypotension), thus affecting the measurement principle.