The twist for circuit boards containing aramid fibers was investigated from the viewpoint of the thermal shrinkage caused by their fiber orientation. Fiber orientation was measured by microwave method, which is based on the dielectric anisotropy. It was confirmed that the fiber orientation could be estimated from the dielectric anisotropy of samples. The dielectric anisotropy measured by microwave method for uniaxially oriented glass fibers was in good agreement with the law of mixture, which shows the appearance of dielectric anisotropy caused by the shape of island (fiber) in the islands-sea structure. On the other hand, the ratio of coefficient of thermal expansion against the orientation degree by microwave method was studied by thermomechanical analysis. Calculated results by our proposed model for the twist of circuit boards were in good agreement with values measured on Japanese Industrial Standard. In order to minimize the twist of two-ply circuit board, the orientation degrees should be minimum or orientation angles of two ply should be the same.