In high voltage (HV) ICs, the latch-up immunity of HV devices is often referred to the TLP-measured holding voltage because the huge power generated from DC curve tracer can easily damage HV device during measurement. An n-channel lateral DMOS (LDMOS) was fabricated in a 0.25-mum 18-V bipolar CMOS DMOS (BCD) process to investigate the validity of TLP-measured snapback holding voltage to the device immunity against latch-up. Experimental results from curve tracer measurement and transient latch-up test show that 100-ns TLP underestimates the latch-up susceptibility of the 18-V LDMOS. By using the long-pulse TLP measurement, snapback holding voltage of the HV device has been found to degrade over time due to the self-heating effect. As a result, since the latch-up event is a reliability test with the time duration longer than millisecond, TLP measurement is not suitable for applying to investigate the snapback holding voltage of HV devices for latch-up.