This study was performed to evaluate the dynamic behavior of turbo pump unit. The acceptable separate margin of $1^{st}$ critical speed was obtained by the use of elastic-ring inserted ball bearing, while the poor separate margin of $1^{st}$ critical speed was appeared in the case without the elastic-ring. In addition, the results show that the stiffness and damping of plain seals give more separate margin of $2^{nd}$ critical speed. However the wear or the failure of seals could reduce the $2^{nd}$ critical speed near the operating speed.