To understand the dynamic of Nothofagus forests in Argentina's Northern Patagonia, the behaviour of some fundamental biological laws related to stand density and mortality were studied. This paper presents the results of applying the law of -3/2 or self-thinning rule to pure and mixed stands of Nothofagus species. Although there are many equations related to this law, it was decided to mainly work with the potential relationship between number of trees and the mean diameter of the stand (Reineke 1933). Also, the mean volume and number of trees relationship was examined. Data from temporal and permanent sample plots were used. For the different forest types, different equation adjustments were made with two different methods: regression and graphical analysis. Two maximum linear trends for size-density relationship were obtained: the fisrt one up to a diameter of 30 cm and the second one over this size. The equations adjustment by means of two methods resulted in a steeper slope than the universal - 1,605. The stands containing N. nervosa displayed the lowest maximum density line. The behaviour of the law was characterised. Also, the Relative Density Index for Nothofagus forests was determined.