It has long been that the unit of oceanic salinity changed from permil (%o) to other unit. However, the middle-and high- school textbooks of science and earth science have still used %o as salinity unit that was defined a long time ago. The objectives of this study are to briefly discuss about the historical development of change in salinity unit and measurement techniques, to present differences between the salinity units of psu (practical salinity unit) and %o, and to address the need and validity for the correction of salinity unit in the textbooks. Twenty-seven kinds of textbooks based on the 7th National Curriculum were analyzed to investigate the expression of salinity unit and the definition of salinity. The results were compared with the usage of salinity units in the articles published in Journal of Korean Society of Oceanography from 1967 to 2008. The percentages by the use of %o were 96.3% in the text and 83.8% in the graphs or tables of the textbooks. By contrast, the scientific papers began to use psu from 1994 and then %o has seldom been used since 2004.