Using a three-dimensional numerical simulation model of lava flow, we studied the effects of parameters on the morphologies of simple flood basalt lava flows. The model can provide a more realistic description of morphologies of lava flows than two-dimensional models because it can evaluate the complex relationship between erupted volume and length of lava by considering lateral spreading of flows. Moreover, our model can calculate lava morphologies on a very moderate slope or even on a flat plane without mesh direction dependency. It can simulate not only small-scale flows on individual volcanoes but also large-scale (>100 km) flows without numerical instability. Although the length of a lava flow is controlled by many parameters including rheological properties and eruption rate, the length is not a simple function of these parameters. We confirm that small changes in parameters such as slope angle and viscosity can affect the length of the flow. Increasing the eruption rate up to 107 m3/s makes basaltic lava flows longer. However, at eruption rates higher than 107 m3/s on 0.3¿ slopes, flow length does not increase beyond that of the 107 m3/s eruption rate due to volume conservation of erupted lava. Thus eruption conditions cannot be estimated from the length of a lava flow alone. Therefore the estimated eruption rate obtained by previous two-dimensional models, which essentially estimate eruption rate from the length of the flow, will not always reveal useful values. On the other hand, the width of a simple lava flow is a good measure of its eruption rate because the width is mainly controlled by the eruption rate and yield strength. A simple lava whose width is greater than 100 km was almost certainly emplaced at eruption rates higher than 107 m3/s if the yield strength of the flow is in a basaltic range. ¿ 1998 American Geophysical Union