We propose invariant, i.e., trace of the square of reduced density matrix, to determine the entanglement pattern of n-qubits state. The substate is separable from its complement substate if the invariant of this substate equals one. Meanwhile, the substate is entangled if the invariant is less than one. Only two types of states that the number of evaluations can be determined. Firstly, a completely separable state can be determined after evaluating n-1 invariants of reduced single qubit. Secondly, a completely separable state can be indicated after 2n-1-1 consecutive calculations of invariants, which started from reduced single state, then followed with reduced double state, reduced triple state, etc., where each invariant is less than one. On the other hand, the number of evaluations of invariants cannot be determined and depend on the pattern of the compound entangle state.