Many weed species are introduced in new region by accident and some of them become component of natural flora. In order to introduce invasive weed species and influence of agricultural practices on weed flora, the results of two surveys, which were performed in 1976 and 1999-2000 in Amasya province, were compared. As 23 weed species were recorded in the first survey, 87 weed species were identified in the second survey. According to the density, the most common species in the first survey were Chenopodium album, Amaranthus retroflexus, Convolvulus arvensis, Heliotropium europaeum and Solanum nigrum. In the second survey, it was found that Xanthium strumarium, Cirsium arvense, Sinapis arvensis, Galium aparine and Bifora radians have gained more importance over a period of 25 years. It was assumed that use of the same weed control methods, rotations, contaminated crop seed and fertilizing greatly altered weed species composition.