隨著近年來個人數位助理(PDA)之盛行，其方便攜帶的特性及多元化的功能，取代了許多過去在電腦上才能執行的作業。因為PDA螢幕尺寸大小受到一定的限制，故其操作績效和視覺負荷需要加以評估。本研究共分為兩部份，第一部分探討不同螢幕閱覽方向和字體大小對於PDA閱讀作業的視覺疲勞和閱讀績效之影響；第二部份探討不同螢幕閱覽方向和字體大小對於PDA搜尋作業的搜尋績效之影響。結果顯示，在閱讀績效與視覺疲勞部份，使用 20pt字體閱讀能有效的降低視覺疲勞的產生，但閱讀速率較使用14pt字體慢，使用橫式閱覽搭配14pt字體閱讀之閱讀速率最快；在搜尋績效部份，使用20pt字體搜尋之搜尋速率較快。最後本研究建議在橫式閱覽搭配14pt字體的環境下進行PDA閱讀作業以及在橫式閱覽搭配20pt字體的環境下進行PDA搜尋作業，會有較佳的作業績效。

PDA's portability and multi-functionality have helped it gain popularity in recent years, and PDA has taken over many operations that used to be the domain of computers. Because PDA has screen size constrains, the resultant user visual fatigue and the inherent limitation on operating performance must be assessed. This research is divided into two parts. The first part studies the effect of screen style(i.e. landscape and portrait) and font size (14- and 20-point) on reading performance and visual fatigue in PDA reading tasks; the second part studies the effect of these same screen style and font size on performance in searching tasks. The results in the first part show that, (1) using 20pt font has significantly lower visual fatigue, however, it also has lowered the speed, and (2) using 14pt font size in combined with landscape allows the highest reading speed. The results in part two show that (1) using 20pt font allows higher searching speed. This research suggests that 14pt font and landscape style would be a better combination for reading tasks, and using 20pt font in landscape style should be recommended for searching tasks.