1. A method for generating a time-domain state space model of an interconnect or transmission tree or net for simulation, performance analysis, model reduction, or circuit design, including the steps of: forming the tree or net as predetermined branches connected as topology of said tree or net; forming branch models and connection models, wherein the connection models are selected from root-leaf connection, or leaf-root connection, or leaf-leaf connection models, or a combination thereof; forming a non-connection model, if there are branches not directly connected among said branches in said tree or net; wherein the models are block matrices; and building the state space model based on the above formed models according to the topology of said tree or net by utilizing a computer or a chip.