Low productivity of rural labor, which is likely to be surplus, is a common feature of agriculture in many countries, especially in developing countries. In Vietnam, rural labor can be used for both on- and off-farm activities. As a resource, farm households want to allocate their labor in productive sub-industries. The use of mathematical models to describe the interactions of variables is useful in modern management. In this paper, a ‘mixed’ model combining the knapsack problem and a household model into a modeling framework is developed. The impacts of changes in land policy (maintenance of rice fields for food security), increase in a minimum wage rate, the output prices of paddy crops were examined in the model. The simulation has led to the conclusion that a policy for sustainable development of agriculture should be considered and required to be adequate.