This paper presents experimental investigations of the characteristics of synthetic jets with a circular exit array. The flow characteristics of piezoelectrically driven synthetic jets with a circular exit array were investigated under various flow conditions. Step-by-step experiments were conducted to find an improved configuration of a circular exit array while some parameters were held constant, in this case the oscillation frequency, input voltage, and exit area. Comparative studies were conducted to compare a quiescent condition, a forced separated flow, and separated flows over high angles of attack. For the quiescent condition, jet characteristics depending on the hole perimeter and oscillation frequency were compared by measuring velocity profiles by means of hot-wire anemometry. For the forced separated flow, pressure distributions on an inclined flat plate were examined while changing the parameters of the hole diameter, hole gap, type of synthetic jet array, and oscillation phase. Experimental results were then analyzed with a help of the computed vortical structures in the quiescent and forced separated-flow conditions. Based on the comparisons, a proper range of design parameters for an improved circular exit array was obtained, showing that the circular exit array based on the design parameters provided better performance in terms of separation control. For separated flows over an airfoil and a blended wing body configuration at high angles of attack, the flow control performance of the proposed synthetic jets was verified by measuring the aerodynamic coefficients. Based on various comparisons, synthetic jets with the improved circular exit array were found to be effective for control of the flow separation phenomenon.