The present work performs one-dimensional simulations of pulsed nanosecond plasma assisted ignition (NPI) at low pressure (80 Torr) in preheated H2-air mixtures in a plane-to-plane geometry. Thermal ignition (TI) is also studied in the same configuration by creating a hot-spot at the center of the computational domain and tracking the progress of subsequent reactions towards the boundaries. NPI requires significantly smaller minimum ignition energy (MIE) than TI due to radical production. Furthermore, NPI is essentially homogeneous while TI requires diffusion of both radicals and heat. The ignition mode of TI actually falls between homogeneous ignition (in which diffusion is not important) and flame propagation (diffusion is dominant). It is essentially consecutive auto-ignition at different locations. Large amount of radicals is first generated inside the hot-spot under high temperature. Then those radicals diffuse rapidly to other locations to trigger the auto-ignition there. From the global perspective, NPI can ignite the entire volume faster than most cases of TI except the case with very small hot-spot radius and extremely high heating temperature. For hot-spot TI, species diffusion transport is found to be as important as thermal diffusion/conduction transport at low pressure conditions.