A recovery facility is important for enhancing the usability of human-computer interaction systems. During recovery operations the user may want to select any command in a recovery history to undo or redo. Many nonlinear undo models have been proposed for such operations. The user may also want to select any object and recover its state. Unfortunately, most undo models do not allow such operations. The authors propose the object-based nonlinear undo model which allows both operations. The recovery history of the model consists of a main history and subhistories. The main history represents the sequence of interactive cycles. The subhistories record the operations on corresponding objects. They also propose a nonlinear undo mechanism which is based on the concepts of active and inactive commands and the rules that determine a command to be active or inactive. They implement the model on the platform of TRIBASE.