本文建立一個搜尋均衡模型，其中不同工作能力的勞動者將依其能力高低選擇居住在不同規模的城市或是郊區中。我們發現勞動者將會選擇居住在與其能力相當的城市中（也就是該勞動者之工作能力接近該城市所有勞動者的平均工作能力）。具高工作能力的勞動者，雖然會冒著較高的失業風險，但卻受到高工資率的吸引而前往能提供高工資率的高生產力之城市。在一個穩定狀態的均衡下，高生產力的城市之城市規模將大於低生產力的城市；此外，城郊工資率的差異也會隨著城市的規模變大而變大。在本文的模型中，城市之數目是內生決定的，且其均衡解不但唯一，也是社會最適解。如果考慮到公司對工作難度的要求所產生的勞動市場摩擦，對城市規模、城郊工資率差異及城市數目的影響將變為不明確，但若是勞動市場摩擦性問題不嚴重，本文之結論則不會改變。在缺乏個別勞動者工作能力的資訊之情況下，對工作難度有不同要求（生產力高之公司之工作難度要求也高）的公司而言，則可透過區位之選擇，以城市規模大小作為篩選機制，找到適任之勞動者。 In this paper, we construct a search equilibrium model in which heterogeneous workers sort themselves into rural and urban areas according to their abilities. We find that a worker tends to search for employment in a city where his ability is close to the average ability of workers in the city. Thus a worker with high ability goes a city that is more productive because of a high wage rate, taking the risk of being unemployed at the same time because of a tight local labor market there. In a steady-state equilibrium, we found that productive cities are larger than those less productive and that the wage gap between urban and rural areas increases with the city size. We also show that the number of cities is endogenous and that the steady state equilibrium is unique, as well as a social optimum. With friction in the part of firms with different job requirements, the impacts of mismatch on city size, wage, and the number of cities become ambiguous in the resulting equilibrium, but remain the same if the market friction is not so severe. Without the information about individual worker's ability, firms with different job requirements (more productive firms require higher job requirements) can find workers with corresponding abilities through locational choices by using city size as a screen device.