Magnetic properties of the high pressure (HP) phase (VO)₂P₂O₂ was studied. It has an unique kind of spin-1/2 Heisenberg alternating antiferromagnetic(AF) chain, whereas its ambient pressure (AP) phase comprises two crystallographically different chains. Magnetic susceptibility, high-field magnetization and heat capacity of the HP phase were measured, and existance of a single spin gap was confirmed. In addition, differential thermal analysis under high pressure was performed to probe the melting point of this HP phase, and its single crystal was grown. Powder XRD under high pressure revealed that there exist more other HP phases.