This paper intends to clarify block formation and building types of Pettah, Colombo, Sri Lanka, and analyze their transformation process. The research on which this paper is based is the further study of the project called 'Field Research on Origin, Transformation, Conversion and Conservation of Urban Space of Colonial Cities', which the major targets are Dutch colonial cities all over the world. Further to this project, comparing colonial cities in South Asia, in terms of spatial formation and transformation is the ambitious objective of this research. Another purpose is to elucidate the Dutch way of houses construction in their colonial cities, where in Pettah. Dutch developed the form of town houses for high-density living. Colombo was once a head quarter of Ceylon, which was one of five administrative districts laid by Dutch East India Company (VOC) on South Asia. Pettah was planned for a city centre of Colombo as a residental area for European citizens. Pettah also played important roll as a commercial centre in British period when the city experienced rapid growth. At present, a large part of town houses are reconstructed. However its street network, block formation and plots are still remaining. This paper clarifies transformation of spatial formation of Peuha from Portuguese ruled period to today, highlighting Dutch ruled period. First of all, this paper illustrates the following topics: the spatial structure of the former Portuguese fort, the construction process of Colombo fort by the Dutch East India Company (VOC) and its spatial structure, and its influence over the British ruling period. In addition, this paper also examines the influence of Simon Stevin's plan of the 'Ideal City' on the spatial formation of Colombo fort. Simon Stevin, was one of the most famous scientists in Europe in the Seventeenth century. Simon Stevin had great influence over military engineering and city construction in his own country. Finally, based on our field survey, this paper clarifies the spatial organization of the present Pettah by analyzing its block formation and distribution pattern of facilities. The authors suggest that the Dutch colonial town planning is still playing a critical role in the spatial formation of Pettah
