The basic structure of human immunodeficiency virus type 1 (HIV-1) has been investigated morphologically; however, the internal structure of HIV-1 core is not well understood. We studied the internal structures by transmission electron microscopy. We modified the method for electron staining of ultrathin sections and processed electron microscopic photographs using a computer. We confirmed that a mature HIV-1 particle had two copies of RNA strands in a cone-shaped core. These two RNA strands formed a coiling structure and interwound each other, and were already present in the late budding stage.