Slenderness is an important issue in design of reinforced concrete (RC) columns. Especially for long columns, second order effects may be not so small to neglect, but the calculation of second order effects may take too much time. For that reason, ACI 318 design code includes a simple approach in order to increase the flexural moment of columns according to their slenderness. Thus, second order effects are considered. In optimization, the effect of slenderness can be considered by using the factored design flexural moments. In this paper, harmony search (HS) algorithm is employed to find the optimum design variables of slender RC columns. These design variables are web width, height, diameter and number of reinforcements. The optimization objective is total cost of materials including concrete and steel. The developed method is effective to find the optimal design for axial force, flexural moment and shear force values. As numerical examples, optimum design of columns with different lengths, but with the same loadings and material properties were investigated. Thus, the effect of slenderness was seen on the optimum costs. By the increase of column length, increase of total material cost is more than a linear increase. This situation shows us the effect of slenderness on optimum RC columns This paper proposes an optimization methodology for reinforced concrete (RC) columns considering slenderness. The proposed methods is used to find design variables of the structural members. In classical design of the engineer, cross-sectional dimensions are assumed and the required reinforcement area is calculated in order to choose design reinforcements. Since steel and concrete have different costs, the most economical design can be only provided by using optimization methods. Design requirements and methods given in ACI318 [1] are considered in optimization process. A metaheuristic algorithm, music inspired harmony search method developed by Geem et al. [2] is employed in the optimization methodology. Several metaheuristic methods have been employed in the design of RC members [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21].