In the case of study on the seismic behavior of tanks, one of the most important subjects is selection of convective and impulsive frequencies for storage tanks. These two frequencies are defined by Housner. The major utility of these frequencies is for Rayleigh damping which is a must for time history analysis. API 650, EUROCODE 8 have suggested some analytical solutions for finding convective and impulsive frequencies and other parameters. In addition, these frequencies can be obtained from modal analysis by finite element software. In current study, these frequencies are obtained using Finite Element (FE) software and performing modal analysis. In the next stage, these modes are compared with analytical methods from standards. To compare tanks with different (H/R) ratios, all these tanks are modeled with the same volume and weight. This study showed that there is a good agreement between convective frequencies extracted from Finite Element method and those from standards. Furthermore, an increase in liquid level led to a rise and dip in convective and impulsive frequencies, respectively. In addition, results show that a 1.47 times increase in the viscosity leads to a 17 percent decrease in the values of impulsive frequencies.