The effects of castling on natural frequencies in 3D steel frames were investigated in this study since accurate assessment of fundamental period (frequency) of a structure is an essential part of design and analysis processes. The use of LUSAS 14.03 was adopted to calculate free vibration characteristics of frames. Thin shell element was used to modeling frames. The initial results are verified by Cinitha.A, Rama Raju. K & Nagesh.R. Iyer (2008) and hand calculations using UBC code.. Furthermore different parameters such as Number of stories, Height of frames, Percentage of Castling, stiffness of connections and Supports Conditions were considered in order to evaluate their effects in frame modal respond. The results of above investigation show that by increasing the percent of castling the natural frequency will increase. Also by reduction of connection rigidity fundamental frequency will reduce. In order to achieve effective earthquake design we also have to consider these two parameters. Finally some suggestions were made in order to improve steel design against earth quake effects.