Some of the cities of northern Iran, such as Sari, Babol, Gorgan and Rasht were regarded as the most important commercial centers for the north of country during past times especially Qajar period. As the case maybe, transportation, shopping and selling agricultural and commercial items and bringing about business type relationships with northern neighbor countries and tourism hold a significant role in the economy and culture of this region. Besides being a market in addition to a shipping center, Downtown caravansaries were also guest houses and residential places for caravansary owners and other people. This research aims in studying downtown caravansaries in northern regions especially Mazandaran and the effective factors for their formation and architectural structure. The major issue in this research as a question can be introduced as this; what is the major characteristic of the wet and mild regions caravansaries in Iran and what are the factors impacting on their formation and architectural structure? What is the reason for the growth of downtown caravansaries during Qajar period and their collapse in contemporary time? This research has been implemented during 2015-2019 and in accordance with analytic-historical method. The required information have been gathered and analyzed through library studies and field review. The field review for the old contexts of cities such as Sari, Babol, Gorgan and their placement in new urban maps and the old aerial picture were performed after library studies and identification of the Mazandaran caravansaries in different documents. Performing interview with market people and the old shop holders was very helpful in order to find their location and investigating their existing situation. The architectural maps for some of the existing caravansaries in Sari and Babol were prepared and documented for the first time and they were suggested to the Mazandaran Heritage for national registration. The central yard together with rounded chambers shaped the architecture of these caravansaries or halls all around Iran. Ceiling coverage type, native materials, decorations, numbers, pop-up types, building establishment type, all of them were influenced by the mild North climate and the Qajar architectural specifications. These halls typically have been built in two floors and commercial chambers, wharfs, and stable in ground floors and resting room for caravansaries’ people were located in the upper floor. After the influence and spread of the Industrial Revolution in Iran, especially in the first Pahlavi period, using automobile for goods transferring instead of four-footed animals, urbanism, establishment of new streets, new form of stores and finally the modern business methods after the influence of the ubiquitous of industrial revolution in Iran, made their usage less colorful which led to the destruction of the most caravansaries. The semi-alive body of some caravansaries are being used after some main modifications, as warehouses for merchants, traditional workshops of different jobs. Some of them also have changed to the two floors commercial halls in the major street corner. In order to pave the way for their conservation, the only solution is recording them as national heritage, as well as restoring and granting modern usage for caravansaries’ contribution in the urban modern life.