Mudflow is one type of natural disaster. This type of mass movement can be triggered by changing of water content due to increasing of rainfall intensity. The occurrence in Indonesia and Taiwan is relatively high. According to its definition, the material is dominant by fine grained soil. The viscosity as one of main rheology parameters governs mudflow movement. In order to overcome the conventional viscometer which could not measure around liquid limit, the authors developed a new laboratory test so called the flow box test (FBT). The governing equation is a couple of Terzaghi’s trap door and Bingham model. From this test, one of the merit is this test can measure the viscosity around liquid limit. Using the case study in Indonesia (Ciwidey and Karangayar) and Taiwan (Maokong), the result shows that increasing of clay fraction is followed by increasing of viscosity. Hence, this research has a contribution to explain the influence of clay fraction on viscosity in relation to mudflow.