A new type of Rotman lens is presented for micro/millimetre-wave applications, such as collision avoidance radar. A dielectric slab Rotman lens is proposed to reduce the conductor loss in addition to the mutual coupling in a form which can be implemented easily at millimetre-wave frequencies. The proposed lens consists of a dielectric slab and slot lines whereas conventional lenses are constructed with parallel conducting plates. The dielectric slab excited in TE0 mode shows a high degree of confinement for the fields, and low dispersion, and has an appropriate feed structure for the dielectric slab lens. A prototype lens was designed and fabricated with nine beam ports and nine array ports together with nine tapered slot antennas. This lens system was tested in the range from 10 to 15 GHz and the measured beam widths are about 15° at 13 GHz. The measurements also show that the mutual coupling between the beam ports decreases with frequency and its efficiency is about 34.6%. The overall performance is comparable to that of conventional Rotman lenses even though a spillover loss is presumed at the frequency of the implementation. It is expected that at millimetre-wave frequency the dielectric slab Rotman lens will have lower conductor loss and lower mutual coupling than conventional Rotman lenses.