Fundamental period is an important parameter in seismic design and performance assessment of buildings. Hence, comprehensive and detailed investigations of effectiveness as well as affectability of this parameter can result in the design of high-performing earthquake-resistant structures. On this basis, this research intends to evaluate the effects of variations of mass and stiffness on the fundamental periods of two three- and nine-story structures representing low- and high-rise buildings, respectively. To this end, a MATLAB code was developed and validated to determine the fundamental periods of structures with various mass and stiffness characteristics. Numerous case studies were performed to investigate the effects of mass and stiffness variations along the stories of the considered structural models. The objective of this research endeavor is to provide a better understanding of affectability of fundamental period under different design considerations.