Urban blocks and road networks are the basic spatial elements which compose urban form. Since urban blocks are the geometrical inverse of road networks, urban block patterns are regarded as the results of plane tessellations by road networks. In recent years, central government of Japan has promoted the land readjustment projects in order to meet the demands for large building lots in mega cities such as Tokyo. In the process of land readjustment projects, small urban blocks (less than 2,000 m²) are merged into a large urban block. However, we lack the theoretical basis for considering the effects of micro scale design (e.g. the edge length of an urban block and the length between the two consecutive intersections) on the macro scale design (urban block patterns and urban block sizes). This is the substantive motivation of this article.
 In the theory and practical applications of urban planning, complete grid patterns of road networks and urban blocks have been considered as the normative patterns. However, on the basis of complete grid patterns, it is impossible to analyze the relationship between the patterns of road networks and urban blocks and variation of urban block sizes. Although, as the results of land readjustment projects, patterns of road networks and urban blocks are changed, their effects on urban block sizes have not been investigated theoretically in the literature. Our research objectives are, by relaxing the complete grid patterns, 1) to investigate theoretically the effect of variation of urban block patterns on the variation of urban block sizes; and 2) to provide the theoretical basis for considering the micro- and macro-scale design mentioned above.
 In this article, we model urban block patterns based on 4 by 4 square-grid block patterns called a region, by relaxing the following two conditions: 1) all intersections are intersections of order 4 and 2) the intersection of North-South and East-West major roads is the center of this region. It is found that 1) urban block sizes follow a log-normal distribution; 2) under the latter condition, relaxing the former condition, decomposing a intersection of order 4 into two intersections of order 3 and making their locations random variable, makes the variation of urban block sizes smaller; 3) by relaxing the latter condition, making the location of major roads random variable, makes the variation of urban block sizes smaller. The last finding implies that a large scale planned road networks tends to make the variation of urban block sizes larger than unplanned ones.
 Also, as the result of empirical analysis based on the size distribution of urban blocks, it is found that, if average urban block size (gross) after readjustment projects is targeted to 10,000 m², by setting the minimum criteria of the edge length of an urban blocks as 50 meters, almost all of urban block sizes are more than 2,000 m². These theoretical and empirical findings contributes to provide the theoretical basis for considering the micro- and macro-scale design mentioned above.