Infrared radiances of sky background and cloud background at night were measured by the medium wave and long wave infrared thermal imager.medium wave infrared radiance and long wave infrared radiance of cloudless sky background are 0.58~1.30 W·m-2·sr-1,6.61~15.29 W·m-2·sr-1 respectively.Because of no influence of solar radiation at night,sky background radiances in all directions are almost same under the condition of the same pitch angles.From the cloudless area to the cloud area,the radiance gradually increases with the increase of cloud thickness.Compared with cloudless background,central radiance of medium wave cloud background increases by 1.5%,and central radiance of long wave cloud background increases by 14.2%,so the effect of long wave band clouds on sky background radiance is more significant.