Abstract: Dissolution of rock fractures is common in natural processes and engineering practices, in which gravity plays an important role. Visual observation technology is a key means to study the mechanism of dissolution in rough fractures. However, the traditional visualization technologies have some problems, such as difficult to reproduce the rough wall of fractures and difficult to observe the dissolution process in real time. In this work, a flow-visualization system for dissolution processes in rough fractures is developed, on which flow-through experiments are conducted on four flow rates (0.05, 0.1, 0.3, and 1 mL/min) for vertical and horizontal fractures, and the gravity effect is evaluated on dissolution patterns and dissolution morphologies, which are characterized by fractal dimension and other morphological parameters. The pore volumes at breakthrough are calculated for each Peclet number (Pe). The experimental results show that the gravity effect significantly influences the dissolution patterns for Pe≤62.1. The dissolution morphologies exhibit buoyancy-dominated patterns and channeling patterns for vertical fractures, and the gravity effect will induce a single and concentrated channel. For horizontal fractures, the dissolution forms dissolution channels with relatively uniform aperture evolution and large width, which are classical wormhole patterns. When the Peclet number is large (Pe=207.0), dissolution in both the vertical and horizontal fractures develops into uniform dissolution. The experimental results also confirm that the dissolution in vertical fractures is more likely to develop channels through the inlet to the outlet to accelerate the breakthrough; Pe=20.7 is the optimal injection condition, which means that the amount of injected liquid is the minimum when breakthrough takes place in vertical fractures. Under this condition, the pore volumes at breakthrough in vertical fractures are only 1/4 of the horizontal fractures. The results in this paper are of great significance to engineering practice such as CO₂ geological storage, indicating that the effect of gravity on the dissolution process should be greatly taken into account.