Interaction with small displays, from ultramobile PCs and smartphones to head-mounted displays, has become an integral part of the daily lives of many technology users. However, compared to typical desktop displays, the small displays on these devices generally occupy small portions of the user's natural FOV, resulting in small viewing FOVs. Additionally, the distance between the eyes and the screen of a smartphone or ultramobile PC can change depending on how the user holds the device, which could further reduce the FOV of the display. Given these changes in display FOV, the same target size/width (W), distance/width (D), and control display gain in pixel or millimeter measurements may be perceived as significantly different, and this difference may affect performance of the pointing task. A similar effect would also occur when using head-mounted displays with different FOV optical systems. Motivated by this, we decided to test whether a smaller display FOV has a significant effect on user performance when completing basic pointing tasks. If this assumption is true, the additional difficulty resulting from the smaller displayed FOV may influence the applicability of Fitts' law which is modeled only by the difficulty index (ID) calculated as a function of W and D. Specifically, if the law Fitts' law, commonly used to evaluate pointing tasks, is not suitable under conditions of limited viewing FOV, so an appropriate update to Fitts's law would help user experience designers better understand and model the additional difficulty when using devices with Limited FOV such as ultramobile PCs, smartphones and head-mounted displays. In this paper we use both discrete and serial Fitts pointing tasks to understand the detailed quantitative relationship... at the center of the paper... of W and D in proportional scale and between the three display sizes (6 × 8, 12 × 16 and 18 × 24 cm), that the smaller display size resulted in a significantly longer MT and a higher error rate. However, to the best of our knowledge, the detailed relationship between screen size (screen FOV) and pointing task performance is not yet fully known. Works Cited Taveira, A.D., & Choi, S.D. (2009). Review study of computer input devices and older users. International Journal of Human-Computer Interaction, 25(5), 455-474.Weaver W. and Shannon C.E. (1959). The mathematical theory of communication. University of Illinois Press Urbana.Zhai S. (2004). Characterizing computer input with Fitts' law parameters: The informative and non-informative aspects of pointing. International Journal of Human-Computer Studies, 61(6), 791–809. Zuckerman J. (1954). Perimetry. Lippincott, Philadelphia.