The formation and evaluation of total trihalomethane (THMs) concentration were investigated using statistical technique over the period from April 2004 to August 2005. The results of THMs formed during water treatment processes at New Benha Water Treatment Plant (NBWTP) showed that THMs levels ranged between (26 and 70 µg/L) and the average concentration was 60 µg/L. A mathematical model that expresses THMs concentration in terms of initial chlorine dose, total organic carbon, bromide ion, contact time, temperature, algae, and pH is developed using the field study measurements for NBWTP. Another mathematical model that expresses THMs concentration in terms of initial chlorine dose, contact time, temperature, and pH is developed supported by experimental bench scale studies. The predicted THMs concentrations from the two mathematical models were compared with the measured THMs concentrations during the sampling period. The results showed high concurrence between the measured and estimated THMs concentrations. Hence, the difference in the average calculated values and measured values during the study period ranged from 1.8 % to 6.6 %. The model provides reasonable results for most of the functioned variables and can be used to have a rapid assessment of the trihalomethane formation.