This study investigated the effects of monolingual book dictionaries, popup dictionaries, and type-in dictionaries on improving reading comprehension and vocabulary learning in an EFL program. An experimental design involving four groups and a post-test was chosen for the experiment: (1) pop-up dictionary (experimental group 1); (2) type-in dictionary (experimental group 2); (3) book dictionary (experimental group 3); and (4) no dictionary aid (control group). Following training on different types of paper and electronic dictionaries, posttests were given to the participants. Findings showed differential effects produced by the three treatments compared with the control group. Results indicated that the pop-up dictionary group had the shortest average vocabulary searching time, vocabulary and text reading time, and more "look-ups" (p<0.0001) than other dictionary groups. In addition, ANOVA analyses showed that text reading time was significantly longer for the book dictionary group. Reading comprehension and vocabulary learning were higher for the pop-up dictionary group than for other dictionary groups. Furthermore, survey data indicated that popup dictionary participants had slightly more positive attitudes toward dictionary use than the type-in group, and both had significantly more positive attitudes than book dictionary participants. Discussion of findings according to cognitive load theory followed, eventually leading to recommendations for teaching and research.