Modern enterprises often place more employees in a smaller area in order to reduce costs. Also, due to the development of office equipment, there are a lot of items which increase noise, such as telephones, video conference facilities, computers, etc. As environmental conditions of offices have become the most fundamental and important issues to improve work efficiency and productivity, this study aims to find out the present state of speech privacy in open plan offices taking an actual measurement and determines how to improve speech privacy using the acoustic simulation software EASE. The target office is realized in the simulation, and four factors are changed: ceiling material, height of partitions, floor material and masking noise. Therefore, it finds out that the most effective way among architectural influential factors is using a high-absorption ceiling material. Also, the method of using a masking noise is one of the effective ways, but it makes employees so annoyed that it is important to consider it only if it does not make them uncomfortable.