This paper describes design and evaluation of a driving mode decision and lane change control algorithm of automated vehicle in merge situations on highway intersection. For the development of a highly automated driving control algorithm in merge situation, driving mode change from lane keeping to lane change is necessary to merge appropriately. In a merge situation, the driving objective is slightly different to general driving situation. Unlike general situation, the lane change should be completed in a limited travel distance in a merge situation. Merge mode decision is determined based on surrounding vehicles states and remained distance of merge lane. In merge mode decision algorithm, merge availability and desired merge position are decided to change lane safely and quickly. Merge availability and desired merge position are based on the safety distance that considers relative velocity and relative position of subject and surrounding vehicles. When the automated vehicle has far distance from the safety distance of all surrounding vehicles, merge is available and lane change is started. The most proper position to merge needs to be selected in case of unavailable merging. The most proper position is selected by considering the safety distances, accelerations, velocities, relative positions and time to merge. And the longitudinal control is applied to move to desired merge position. A safety driving envelope is defined based on the desired driving mode using information of surrounding vehicles behaviors. To obtain the desired steering angle and longitudinal acceleration for maintaining the subject vehicle in the safe driving envelope, motion planning controller is designed using a model predictive control (MPC) with constraints. The proposed control algorithm has been evaluated via computer simulation studies.