The connections are idealized as either fully rigid or ideally pinned in conventional analysis of steel frame structures. The use of the rigid or pinned connection model in frame analysis serves the purpose of simplifying the analysis and design processes, but there are differences between the idealized behavior and the actual behavior. The actual behaviour that falls between these two idealised models has been classified as semi-rigid steel connections. The design and analysis of steel space frames with semi-rigid connections is studied by using a specific computer program, prepared in the MATLAB language. The analysis of three dimensional (3D) steel frames with semi-rigid and rigid connections is obtained using the stiffness matrix method. In the present study, it is designed according to the buckling, combined stress and displacement constraints as specified in Turkish Building Code for Steel Structures under the present loading after it is determined the joints deformations and the member end forces of the space frame. The Frye–Morris Polynomial Model is used to describe the non-linear behaviour of semi-rigid connections. Semi-rigid connection type is limited to top and bottom angle with double web angle connection.The results of design and analysis of unbracedsemi-rigid frames are compared to the results of unbracedrigid frames under the same design requirements. With the semi-rigid connection design, it is yielded in more economical solutions.So, the semi-rigid connections are the actual behaviour of beam-to-column connections in steel frames. Therefore, neglecting the actual behaviour of the connection in the analysis may lead to unrealistic predictions of the response and reliability of steel frames.