Provided are an apparatus for and a method of processing a substrate. The substrate processing apparatus includes a substrate processing unit to process a substrate using a processing solution containing a mixture of first and second sources; a source supplying part to supply the first and second sources to the substrate processing unit; at least one analyzer to measure a concentration of the second source in the processing solution or a pH value of the processing solution and adjust a measurement reference value of the second source in the processing solution using a standard solution, in which the first and second sources are mixed to have a predetermined concentration or pH value; and a standard solution supplying part to prepare the standard solution using the first and second sources to be supplied from the source supplying part and to supply the standard solution to the at least one analyzer.