Models of interaction tasks are quantitative descriptions of relationships between human temporal performance and the spatial characteristics of the interactive tasks. Examples include Fitts' law for modeling the pointing task and Accot and Zhai's steering law for the path steering task. Interaction models can be used as guidelines to design efficient user interfaces and quantitatively evaluate interaction techniques and input devices. In this paper, we introduce and experimentally verify an interaction model for a 3D object-pursuit interaction task. Object pursuit requires that a user continuously tracks an object that moves with constant velocities in a desktop virtual environment. For modeling purposes, we divide the total object-pursuit movement into a tracking phase and a correction phase. Following a two-step modeling methodology that is originally proposed in this paper, the time for the correction phase is modeled as a function of path length, path curvature, target width, and target velocity. The object-pursuit model can be used to quantitatively evaluate the efficiency of user interfaces that involve 3D interaction with moving objects.