This paper deals with the rainwater harvesting technology dissemination process in a small town of arsenic prone coastal Bangladesh. Based on the social network threshold model, we have specified the adopter's degree of innovativeness in respect of system or region level and personal or neighborhood network level to understand the effects of macro and micro level networks on rainwater tank adopters in Morrelganj town. In the study area, as the innovators were sparely distributed, lack of social learning and social pressure could not render the tank adoption diffusion in the initial phase. More than 50 percentage of adopters who adopt when their system and neighborhood level exposure are about the same which presumably indicates the impacts of group norms. Our study has also derived the following implications. The earliest innovators both irrespective of system level and personal level first bring the innovative ideas into the community and then it diffuse in a normative fashion through social interaction. External influence on the adoption decision behavior has not been found so significant, except for the effect of visiting the nearest city, which may suggest that in such a small town mass media have very limited impact on diffusion of innovation ; rather information flows through inter-personal channels are likely to affect the process. Further research needs are mentioned such as data and evidence consolidation as well as other model building.