|Compare to large office buildings, small and medium buildings show differences in characteristics such as purposes, location and type of investors. Unlike large office buildings, small and medium size buildings do not have systematic ways to approach. That is why studies on small and medium size building market have not been done sufficiently. In order to analyze small and medium building market in Seoul area, real transaction data from 2009 to 2015 was collected and the characteristics were examined. Factors influencing Net Operating Income(NOI) was analyzed through regression analysis. These are the main characteristics of small and medium buildings found using real transaction data. First, small and medium size buildings are owned mostly by individuals rather than corporate bodies. Second, small and medium buildings tend to have mixed use purposes and retail takes up the largest proportion. Third, they prefer to be located in the main commercial areas which could be different from main office areas. Forth, the rent price of the 1st floor has great influence on the price of the building. Fifth, small and medium buildings are developed and managed rather unsystematically. There are a lot of buildings that were developed under its possible floor area ratio and management fees are collected in many different ways. These characteristics were reflected in the regression analysis to find out the factors influencing NOI. The determinants of NOI of the small and medium buildings were: elevator installation; 1st floor lease area ratio; vacancy area ratio; existence of residential tenants; building owner`s usage; and management expense payment method. NOI were influenced not only by 1st floor rent price, but also many other characteristics such as underground level usage and 2nd floor and above area usage.