Accurate multiphase flow measurement for gas/liquid, liquid/solid and liquid/liquid flow is still a challenge for researchers in process tomography. The reconstructed images are poor, particularly in the centre area because of ill-posed inverse problems and limited measurement data. Dual-modality tomography has been induced to overcome the problem; each modality is sensitive to specific properties of the materials to be imaged. This paper proposes the combination of ultrasonic transmission tomography (LET) and electrical resistance tomography (ERT) for imaging two-phase gas/liquid. With the combination of hard-field and soft-field techniques, the detection ability of objects in the medium of interest improved because two different images of the same space can be obtained simultaneously. This paper presents both a three-dimensional (3D) and two-dimensional (2D) numerical modelling approach using COMSOL software for ERT opposite excitation strategy and ultrasonic transmission tomography. The investigation of this work is to analyse the optimum electrode size in order to improve the situation of: (1) potential distribution and current density in the medium of interest, (2) sensitivity in the central pipeline area. The developed numerical model simulated the changes in resistivity of the conductive material, owing to the variations of electrode sizes, when opposite current excitation was injected into the region of interest. Simulation results show that an electrode size of 12 mm (w) x 90 mm (h) is suitable, which gives good detection of 10-mm gas bubbles in a 100-mm-diameter acrylic vessel. (C) 2014 Elsevier B.V. All rights reserved.