With the development of communication technology, the precious frequency spectrum is becoming more and more crowded. Exploiting the particular filter with zero group delay, ultra narrow band (UNB) modulation is intended for acquiring the high frequency efficiency. Unfortunately, this UNB filter is confronted with great challenges from the classical communication theories. The validation on its realization is also difficult. This paper proposes a novel UNB modulation, namely random pulse position keying (RPPK). It is demonstrated from analysis that there is no inessential discrete spectral line in modulated signal’s power density spectrum (PDS), and the UNB filter has been successfully avoided. Consequently, RPPK is rather explicit in theory and simple in implementation. As is shown by the simulation, the frequency utilization of RPPK can even reach 100 bits/s/Hz. Its optimum receiving performance is slightly inferior to BPSK, yet is much superior to that of other UNB modulations such as VMSK/2. Moreover, multi-access and confidentiality are additional benefits to UNB users. Exploiting the development of communication technology, the precious frequency spectrum is becoming more and more crowded. Exploit the particular filter with zero group delay, ultra narrow band (UNB) modulation is intended for acquiring the high frequency efficiency. Unfortunately, this UNB filter is confronted with great challenges from the classical communication theories. The validation on its realization is also difficult. This paper proposes a novel UNB modulation, namely random pulse position keying (RPPK). It is demonstrated from analysis that there is no inessential discrete spectral line in modulated signal power density spectrum (PDS), and the UNB filter has been successfully avoided. RPPK is rather explicit in theory and simple in implementation. As shown by the simulation, the frequency utilization of RPPK can even reach 100 bits / s / Hz. Its optimum receiving performance is slightly inferior to BPSK, yet is much superior to that of other UNB modulations such as VMSK / 2. Moreover, multi-access and confidentiality are additional benefits to UNB users.