Provided are an apparatus and method for determining a diameter of a reflector antenna. The apparatus includes a setting unit for setting up electrical characteristic values of the reflector antenna upon receipt thereof from outside, a parameter input unit for receiving a frequency, an antenna diameter variable value, antenna efficiency, Edge Taper (ET), and an Edge Of Coverage (EOC) angle as parameters for each of multiple frequencies to be accommodated in the reflector antenna, a graph generator for generating, on one rectangular coordinate, a graph representing the relationship between an antenna diameter and EOC directivity with respect to the EOC angle based on the parameters for each frequency received through the parameter input unit, an antenna diameter determination unit for determining an antenna diameter that simultaneously meets the preset EOC directivities for the frequencies to be accommodated in the reflector antenna by using the graph generated by the graph generation unit, and a graph output unit for mapping and outputting the graph generated by the graph generation unit on one rectangular coordinate with an antenna diameter axis and an EOC directivity axis to thereby represent the antenna diameter determined by the antenna diameter determination unit on the graph.