Electro-active papers (EAPap) made with cellulose have been known as a biomimetic actuator material according to their merits in terms of large bending deformation, low actuation voltage, ultra-lightweight, and biodegradability. This out-of-plane bending deformation is useful for achieving flapping wings, micro-insect robots, and smart wallpapers. On the other hand, in-plane strains, such as extension and contraction of EAPap materials are also promising for artificial muscle applications since the Young's modulus of EAPap materials is large. This paper first reports the experimental investigation of the in-plane strain of EAPap materials in the presence of electric fields. The EAPap samples preparation and the in-plane strain measurement system are explained. The test results are shown in terms of electric field, frequency and the orientation of the samples. The power consumption and the strain energy of EAPap samples are discussed. Although there are still unknown facts in EAPap materials, this in-plane strain may be useful for artificial muscle applications.