Using data of literacy skills among adults aged 26 to 35 from the International Adult Literacy Survey, we compare the degree of literacy gaps between those who completed tertiary education and those who did not graduate from high schools across 19 countries. The result of ordinary least square regression shows that although those with a higher level of educational attainment tend to have a higher level of literacy skills in all countries, countries substantially vary in the degree of literacy gaps by educational attainment. The cross-national variation in the literacy gap is mainly driven by between-country differences in the level of literacy skills among those who did not graduate from high school. The result of two-level hierarchical linear models, furthermore, shows that the cross-national variation in the literacy gap by educational attainment is in part attributable to between-country differences in standardization of educational systems and the extent to which adult education and training are offered to the low educated. We discuss theoretical and policy implications of the findings for addressing inequality of literacy skills.