The use of plain (smooth and undeformed) rebars as longitudinal and transverse reinforcement in RC members is one of the most encountered practices in old construction technology. Thus, probable vulnerabilities due to plain rebar implementation should be considered in the seismic evaluation of RC buildings. It is known that reduced bond stresses distributed along the plain rebar surface due to the absence of the ribs leads to a resultant reduction in effective stiffness and an increase in rotation capacities for RC members. Therefore, RC buildings that were constructed according to the older design codes using plain rebars were expected to deform laterally beyond the anticipated deformation capacities that were determined based only on the RC members reinforced with deformed rebars. In this study, the modeling strategies were examined and verified with the results from an existing building test in the literature, and then the seismic performance of a five-story RC framed building, which was reinforced with plain rebars, was evaluated under nonlinear time-history analysis (NTHA). In order for code comparison, the nonlinear characteristics of the RC members were determined according to the previously suggested model for members reinforced with plain rebars and the current Turkish earthquake code regulations. The parameters investigated were (1) the number of members at each damage state, (2) the maximum interstory drift ratios, and (3) the normalized story shears by building weight considering the earthquake type (pulse type or non-pulse type) and predamage effects. The analysis results emphasized the importance of rebar characteristics in seismic performance assessment, because the use of plain rebars reduced the number of plastic hinges while leading to higher drift ratios. This phenomenon is analogous to the behavior of RC beams reinforced with plain rebars which fail with a smaller number of wide cracks rather than a greater number of narrow cracks as in the case of deformed rebars.