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<p align="justify"><em>We have utilized the Global Positioning System (GPS) data at 57 stations distributed over Sumatra Island to investigate spatio-temporal variations of the atmospheric precipitable water vapor (PWV). We focused on the annual and semi-annual cycles of the PWV. Our results show that Sumatra Island is divided into two distinct areas of annual and semi-annual cycles, where the boundary line between the areas is </em><em>approximately a</em><em>t</em><em> 2</em><em>^o</em><em>S. While the annual cycle dominates the area over the southern side of 2</em><em>^o</em><em>S, the semi-annual cycle is dominant over the northern side. Our results have further shown that</em><em> the maximum phase of annual cycle </em><em>occurs</em><em> between January-March with considerably large amplitudes (10-15 mm). On the other side, the maximum phase of the semi-annual cycle in general occurs around November and May, whose amplitude is approximately between 1-5 mm. Our results are consistent with other results using rainfall data.</em><em></em></p>