abstract: The geodesic flows on surfaces with constant negative curvature have rich dynamics. Hoph proved that geodesic flows on modular surface are ergodic. One of the tools to extract other dynamical behavior of such flows is to correspond a sequence of non-zero integers to each flow and verify the properties of the obtained space. Here, we introduce an algorithm to find the geometric code of the geodesic flows on modular surface. We give some conditions on which a sequence of non-zero integers is realized as a geodesic flow.