Three standard types of obstacles are selected for the study: vertical step, isolated wall and ditch. When one of these obstacles is detected in the direction of motion, a quadruped reads only one or two parameters of obstacle and then calculates corner distances, which makes it possible to determine the locomotion sequences before it reaches the obstacle. Vertical step is fully defined by one parameter, step height. An isolated wall is defined by two parameters: wall thickness and vertical height. A ditch is defined by ditch width. The dimensions of the suggested model are used to calculate the crossing performance of a quadruped, for the purpose of comparison. All the crossing gaits are illustrated by simulation figures. Based on the results of this research, a more comprehensive study in the walking over a combination of these obstacles can be conducted and a four-legged walking machine which moves with mobility can be designed.