Abstract: This paper is a study on algebraic structure of Pre A*-algebra. First we define partial ordering on Pre A*-algebra. We prove if A is a Pre A*-algebra then (A, ≤) is a poset. We define a semilattice on Pre A*-algebra. We prove Pre A*-algebra as a semilattice. Next we prove some theorems on semilattice over a Pre A*-algebra. We define distributive and modular semilattices on Pre A*-algebra We define complement, relative complement of an element in Pre A*-algebra. We define complemented semilattice, relatively complemented semilattices in Pre A*-algebra. We give some examples of these semilattices in Pre A*-algebra. We define weakly complemented, semi-complemented, uniquely complemented semilattices in Pre A*-algebra. We prove some theorems on these semilattices in Pre A*-algebra.