We report on formation of self-organized nanodots on semi-insulating InP surface due to bombardment by 100 keV Ar+ ions incident both normally and off-normally (0 degrees, 30 degrees, 60 degrees, and 75 degrees with respect to the surface normal) for three fluences viz. 1 x 10(17), 5 x 10(17), and 1 x 10(18) ions cm(-2) at room temperature and without any substrate rotation. The novelty of our work is that we have studied pattern formation, at this energy range, in a systematic manner as a function of incident angle of ions. It is seen that average dot-size, -height, and inter-dot distance decrease with increasing angle of incidence (for a given fluence), while dot density increases. This trend is followed for all the fluences under consideration. RMS surface roughness shows a decreasing trend with increasing angle of incidence. This is indicative of surface smoothening due to enhanced ion induced surface diffusion at higher incident angles. Dot formation is attributed to preferential sputtering. We do not observe any transition from dot to ripple pattern unlike low energy experiments performed on semi-insulating InP substrates. Although evolution of dot patterns on both n- and p-type InP surfaces, at intermediate energies, are known there are subtle differences in terms of variations in dot-size and -density for semi-insulating InP substrates. (C) 2011 Elsevier B. V. All rights reserved.