In modern society, using the Video Display Terminal (VDT) such as a personal computer or a smart-phone is universal that results in the increasing of upper extremity musculoskeletal diseases. The purpose of this study was to investigate the relationship between sternocleidomastoid muscle, upper trapezius muscle thickness and the height of table while the computer working. 15 healthy adult males and 15 females were recruited and agreed to participate as subjects in the study after a preliminary research. After setting 5cm, 15cm and 25cm height difference between the knee and the table, the subjects performed the computer work five minutes for each height while maintaining the working position of the machine and were measured the thickness of sternocleidomastoid and upper trapezius. As a result, there were significant difference for the change of thickness of upper trapezius and sternocleidomastoid muscle that were measured after the working of the computer on the height difference between the knee and the table in 5cm, 15cm and 25cm (p<.05). The thickness of all the sternocleidomastoid muscle and upper trapezius is thinnest at the time of the height difference in 15cm.