Abstract: As of the mid 1990s economic reforms in Brazil impacting on land conflicts have intensified due to uncertainties surrounding property rights on land. The objective of this paper is to study the invasions of land properties held by the so called Brazilian social movements. This conflict concerns the disputes between farmers and social movements in which the landowners questioned the legality of the invasion for agrarian reform and compliance with the immediate mandate of reinstatement of possession when there is invasion of property. On the side of the social movements land invasions have become the main form of pressure to speed up expropriation processes and settlements. Having as institutional landmark the “anti-invasion Law” associated with the report on properties invaded released by the Federation of Agriculture of the State of Parana, we developed an empirical model applied to land owners in conflict areas encompassing invasions occurred over 2000 and 2006. Based on the premises of the New Institutional Economics and on the analysis of land property rights and private investments this study examined the relation between the variable “invasion period” (property right insecurity) and “land productivity level” (investment), by comparing farmers productivity before and after the invasion. Analysis of the research results allowed concluding that the problem of land invasions is expressive and bears directly on the investment decisions made by farmers in the State of Parana.