This study investigated the validity of four theoretically motivated traits of writing ability across English and Korean, based on elementary school students' responses to letter- and story-writing tasks. Their responses were scored analytically and analyzed using confirmatory factor analysis. The findings include the following. A model of writing ability that includes the influence of four primary trait factors (grammar, content, spelling, and text length), which are influenced by a higher-order trait factor, and of the effect of test methods (letter- and story-writing tasks) provides a reasonable explanation for differences in writing performance among students. The trait effects are central while the method effects peripheral and inconsistent. A higher-order factor explains the correlations among the four primary factors, whose uniqueness is retained even while removing the effect of the general factor. This research expands our understanding of writing performance in the following unique aspects: models of writing with the four traits and the two tasks across two languages, largely unverified aspects of writing in prior factorial studies; a CFA-correlated-uniqueness approach for trait-method investigation. (Contains 1 figure, 4 tables and 7 notes.)