Parthenium hysterophorus is a noxious weed, which causes significant threat on agriculture, human health and biodiversity in several countries including Sri Lanka. Since the first report in late 90s the weed spreads aggressively in many regions in Northern Sri Lanka. There is an urgent need to eradicate the weed from infested sites and prevent further spreading. With this background, present study aimed to identify the major threaten sites in Jaffna peninsula. People were asked via newspaper article to inform about the sites which infested with P. hysterophorus. Based on their responds four administrative divisions Valikamam, Vadamarachi, Thenmarachi, and Jaffna town were selected for sampling. Altogether 24 sampling sites were selected from above four divisions. Population density of P. hysterophorus was measured in each site by laying 1m quadrate for 20 times. Population density of P. hysterophorus ranged between 16 to 78 plants / m2 in Jaffna peninsula. Higher infested sites were noticed in Valikamam division and followed by Thenmarachi division (36 plants / m2). Population density of Parthenium plants was less in Vadamarachi and Jaffna town divisions, about 17-18 plants / m2. The findings reveal that Valikamam division is in higher risk due to invasion of noxious P. hysterophorus, therefore, significant actions are needed to manage this weed.