Cadmium selenide (CdSe) nanomaterials of different shapes have been synthesized in the water pool of the water-in-oil microemulsions through 7 MeV electron beam irradiation. The rod shaped CdSe nanomaterials of lengths and breadths up to 1 μm and 200 nm, respectively with aspect ratios from 3 to 5 were grown in the case of microemulsions containing lower water content, water to surfactant concentrations ratio, w0=10. In contrast, the cubic shaped CdSe nanomaterials of dimensions about 100 nm were formed in the microemulsions with higher water contents, w0 values from 20 to 40. Such a transformation of shape from rod to cube was attributed to the change in the hydrophobicity and hydrophilicity of the micro-environment in the microemulsions. The as-grown CdSe nanomaterials were stable at ambient conditions and thus expected to be very useful in the device applications.