We report the structure and properties of segmented poly(urethaneurea) (SPUU) with relatively short hard-segment chains. The SPUU samples comprised poly(tetramethylene glycol) prepolymer as a soft segment and 4,4'-diphenylmethane diisocyanate (MDI) units as a hard segment that were extended with ethylenediamine. To discuss quantitatively the conformation of the soft-segment chain in the microphaseseparated domain space, we used SPUU samples for which the molecular weights of the hard- and soft-segment chains are well characterized. The effects of the cohesive force in the hard-segment chains on the structure and properties of SPUU were also studied with samples of different chain lengths of the hard segment, although the window ofx(H), the average number of MDI units in a hard-segment chain, was narrow (2.38 less than or equal to x(H) less than or equal to 2.77). There were urethane groups in the soft segments and urea groups in the hard segments. Because of a strong cohesive force between the urea groups, we could control the overall cohesive force in the hard-segment chains by controlling the chain lengths of the hard segment. First of all, microphase separation was found to be better developed in the samples with longer hard-segment chains because of an increase of the cohesive force. It was also found that the interfacial thickness became thinner. The long spacing for the one-dimensionally repeating hard- and soft-segment domains could be well correlated with the molecular characteristics when the assumption of Gaussian conformation was employed for the soft-segment chains. This is unusual for strongly segregated block copolymers and might be characteristic of multiblock copolymers containing rod-coil chains. The tensile moduli and thermal stability temperature, T-H, increased with an increase of the cohesive force, whereas the glass-transition temperature, the melting temperature, and the degree of crystallinity of the soft-segment chains decreased. The increase in T-H especially was appreciable, although the variation in the chain length of the hard segment was not profound.