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The nonradiative dielectric waveguide (NRD‐guide) is a dielectric waveguide which normally is composed of low‐permittivity dielectric materials; but, if high‐permittivity materials are used, then the circuit size may be reduced. However, when high‐permittivity materials are used, then single‐mode transmission becomes impossible and some unpredictable phenomena are observed which have never been observed in a low‐permittivity waveguide. In this paper, transmission characteristics of the high‐permittivity nonradiative dielectric waveguide are studied theoretically as well as experimentally. Assuming that the presence of small air gaps between the dielectric strip and metal plates of the NRD‐guide, which cannot be avoided in practice, is the origin of these phenomena, the analysis has been performed which shows that a serious effect occurs even for extremely small gaps and the bandwidth of single‐mode transmission becomes very small. Ultimately, this makes the single‐mode transmission impossible. The electromagnetic field distribution in the NRD‐guide is also calculated and the curious phenomena are interpreted physically. Moreover, a trapped insular NRD‐guide is proposed which permits single‐mode transmission even if high‐permittivity material is used. An experimental confirmation is made of the usefulness of this new type of NRD‐guide.

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