Nonlinear transmission lines (NLTLs) provide a nonlinear, dispersive medium for soliton propagation. Taking advantage of the soliton propagation effects, electrical impulse compression and harmonic generation can be achieved in NLTL circuits. In this paper, characteristics of solitons propagating along NLTLs have been investigated and simulated by the finite-difference time-domain (FDTD) technique. Unique nonlinear interactions among solitons propagating along nonlinear transmission lines are observed in our model. A novel NLTL millimeter-wave tripler has been developed which is realized by periodically loading back-to-back diodes in a high-impedance transmission line. Our simulation results show that this NLTL tripler has higher conversion efficiency than conventional NLTL triplers.