Shadow on the absorber plate of a flat-plate solar collector or a solar cooker box can reduce absorbed energy. The main goal of this research is to investigate the effective factors on the shadow formation inside a solar collector. In addition, the energy gain reduction due to shadow was calculated. The length, width (0.5-2.5) and height (0.01-0.2 m) of a solar collector, tilt angle (0.01-80 degrees) and latitude (0.01-65 degrees) were considered as the effective variables on the shadow formation. The sum of shadow ratio and the percentage of energy gain reduction per year were chosen as the dependent variables, and the effect of each factor was discussed. Results showed that the effective variables (strong to weak) were height, width, tilt angle, latitude and length for the sum of shadow ratio per year, and height, tilt angle, width, latitude and length for the energy gain reduction per year. The minimum and maximum energy gain reduction per year due to shadow for a solar collector with length = width =1 and height = 0.04 m were obtained 5.23 and 21.64%, respectively. If a solar collector has a rectangular plate, the larger width is more suitable than the larger length for the shadow reduction. (C) 2019 Elsevier Ltd. All rights reserved.