When suitable site trees are lacking in a stand, indirect methods are often used to predict site index. One such indirect method is to convert the site index of one species (the present one) into the site index of another species (the missing one) through a site index conversion equation. To develop a site index conversion equation, it is required that site index estimates must be obtained from those stands with both species present. Because of interspecific interactions, this requirement intrinsically violates assumptions behind the site index concept. To solve this problem, an ecologically based model was proposed in this study. Based on data obtained from the Sub-boreal Spruce zone of British Columbia, pure or single species dominated stands of lodgepole pine (Pinus contortaDougl. ex Loud.) and white spruce (Picea glauca(Moench) Voss) were matched within each of the three studied subzones by using the concept of ecologically equivalent sites. The method of reduced major axis (RMA) was used to develop site index conversion equations between the two species. The resulting equation was then compared with the soil-site studies and the site index comparison study on the same two species conducted in the same region. By applying the ecologically based model, site index conversion equations can be developed for any possible pair of species existing in a study area, thus making site index conversion among species operationally feasible based on forest inventory data.