Operation mode dependent series resistance (R-sd) behavior of junctionless transistors (JLTs) has been discussed in detail. R-sd was increased for decreasing gate bias in bulk conduction regime, while a constant value of R-sd was found in accumulation operation mode. Those results were compared to conventional inversion-mode (IM) transistors, verified by 2D numerical simulation and temperature dependence of extracted R-sd. This work provides key information for a better understanding of JLT operation affected by R-sd effects with different state of conduction channel.