Wind tunnel tests are conducted to investigate aerodynamic characteristics of two tandem square cylinders in horizontal and diagonal arrangements at a Reynolds number of Re = 8.0×104 and spacing ratios of P/B = 1.75 ~ 5.00, where P is the center-to-center spacing of the two cylinders and B is the width of the square cylinder. The surface pressure distributions on cylinders in two arrangements are obtained. Efforts are mainly devoted to the aerodynamic characteristics of two diagonal tandem cylinders, and the comparison with those of the two horizontal tandem cylinders. The results show that the mean drag coefficients on the downstream cylinder for the horizontal arrangement discontinuously vary from a negative value to a positive one when the spacing ratio increasing from 3.00 to 3.50, while the mean drag coefficients for the diagonal arrangement remain negative in all spacing investigated. For P/B< 3.00, the mean and fluctuating aerodynamic forces, the maximum of mean negative pressures and the fluctuating pressures of two diagonal tandem cylinders are generally larger than those for the horizontal tandem cylinders, whereas the scenarios are completely contrary for P/B > 3.00. Furthermore, the Strouhal numbers of the diagonal cylinders are significantly smaller than those of the horizontal ones at the same spacing. It is interesting to note that there are multiple peaks in the lift power spectra for the diagonal cylinders for P/B < 3.00.