Abstract: Olive fly, Bactrocera oleae Gmelin (Diptera: Tephritidae), was a quarantine pest inIran before its invasion in 2004. Since then it has been one of the main pests on olive in thecountry. By having a high population in several generations per year, the pest voracious larvaefeed on the fruit flesh and make them fall before harvest. Besides, the harvestable fruits infestedby the pest would have degraded quality of oil due to their vulnerability to rotting by pathogenicfungi. Facing with such a dilemma, a study was necessary to obtain the most valid method of pestsampling considering timing and direction for further pest incidence prediction, damage and croploss assessment and/or control measure decisions. Therefore, a research on the matter was carriedout during 2006-07. First, 5 olive groves, 1h each, were selected in Tarom Sofla, Qazvinprovince. Then, 5 trees were selected randomly in each grove for fruit sampling, weekly. Thesame number of trees was also selected for soil sampling. The samples were taken from four maindirections of each selected olive trees. The results revealed significant differences amongtreatments. Similarly, the mean number of infested fruits was significantly higher in south (4.11fruits/sample) and east (3.91 fruits/sample) directions compared with that of north (3.31fruits/sample) and west (3.21 fruits/sample). Besides, the mean length of fruits proved to besignificantly more in south and east directions compared to that for north and west. Thereby, apositive correlation was established between fruit length and infestation incidence throughPearson Correlation Analysis. The similar results were obtained for the population of pupal stagein soil as its mean number was significantly higher in south (1.03) and east (0.96) directions. Thevarious weekly sampling proved that the frequency of pest incidence (infested fruits) increasedlate in Oct. and Nov. Conspicuously, there were two peaks of pest sampled population, one onOct. 30, and the other on Nov. 21, 2007.