Horizontal heated cylinders were placed at equal intervals to constitute 3 X 3 and 3 X 5 in-line arrangements, and natural convection heat transfer of the arrangements was experimentally investigated in air. Average and local heat transfer coefficients of central vertical array were measured to extract representative heat transfer characteristics of each arrangement. Each heat transfer coefficient was expressed well by using the previously reported correlation methods for a vertical single array (Tokanai et al., 1997) and a horizontal single array (Kuriyama et al., 1995).