This study aims to develop a web-based task that requires the students' participation in authoring of multimedia annotations and to investigate its effects on the learning of English vocabulary and attitudes toward English vocabulary learning of high school students. The subjects of this study consisted of one hundred twenty six first-year high school students. The control group consulted the multimedia English annotation provided by teacher on the internet cafe while reading. The experimental group participated in authoring of multimedia annotation while reading. The subjects had the first vocabulary test immediately and a week later the second unexpected vocabulary test was given to examine the retention of the acquired vocabulary. Right after the second test, the vocabulary test provided with images of the target words was given to examine the effects of image on recall. In addition, the attitudes and opinions toward the vocabulary learning through the multimedia learning materials were investigated by questionnaires and in-depth interviews. The major results of this study are as follows. First, the results showed a significant effect of the authoring of multimedia English annotation on the vocabulary learning. But its effects on the retention was not significant compared with those of the control group. Also the images had a significant effect on recall when they were given in the test. Second, the results of the questionnaire indicated that the attitude toward English vocabulary learning became significantly positive when the students participated in authoring of the multimedia learning material. Although the difference between the control and experimental group was not significant, both groups showed the positive attitudes. Third, the results of the additional questionnaire and the interview indicate that in general the experimental group students were satisfied with the task. The learners generally made good use of English annotation to consult and find the exact meaning of the target words. They also answered that this task increased interest and intrinsic motivation through the activities choosing appropriate images and listening to the pronunciation on the computer. Moreover they evaluated the task as effective way of learning vocabulary due to the active student involvement in the learning process. In summary, the results of this study indicate that the task of authoring of multimedia English annotation has positive effects on not only vocabulary learning but also the students' attitudes toward English vocabulary learning. This suggests that the effective tasks using multimedia should be provided to encourage the students' active mental process to link the forms and meanings of the words.