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 4,4Ј-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 of x H , the average number of MDI units in a hard-segment chain, was narrow (2.38 Յ x H Յ 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.