The aim of present study is to assess the impact of atrium width and clerestory height on amount of Average Daylight Factor (ADF) in different floors of vertical top-lit atria and determining the appropriate geometrical sizes for the four-storey, four-sided atrium to provide sufficient daylight in office spaces. The ADF_s predicted using the modelling software IES-Radiance are validated by scale model measurements. Study on the relationship between atrium width (W) and daylight availability in the adjoining spaces of atrium indicates that, the variations trend of ADF in the different floors of atrium are not similar; as the maximum ADF occurs when the width of atrium provides an optimum distance between the office rooms in each floor and the clerestory opening; this distance is significantly related to the sky view angle, altitude angle and optimum distance that daylight can penetrate in the office rooms. Furthermore, it is found that growing the height of clerestory windows increases the amount of ADF in the atrium and its adjoining spaces. It concluded that the minimum acceptable ratio of clerestory height (h) to atrium height (H) for providing the sufficient level of ADF in the atrium adjacent spaces is h/H=3/8.