The influence of temperature on the critical in-plane field range for vertical Bloch lines in the walls of three kinds of hard domains is investigated experimentally. It is found that for each kind of three hard domains, there exists a critical in-plane field range, i. e. [Hip(1)(T),Hip(2)(T)], which depends on temperatures and in which vertical Bloch lines are unstable. Here, Hip(1)(T) is the initial critical in-plane field where VBLs in the walls of three kinds of hard domains are annihilated, and Hip(2)(T) is the lowest in-plane field where VBLs in their corresponding hard domains are annihilated completely. Hip(1)(T), Hip(2)(T) and [Hip(1)(T),Hip(2)(T)], all decrease as the temperature increase. Furthermore, Hip(1)(T) and Hip(2)(T) reach zero at T01 and T0, respectively. In addition, there exists a relationship among them, when T is unchanged, Hip(1)(T) of the three kinds of hard domains (ordinary hard bubbles (OHB), first kind of dumbbell domain (ID) and second kind of dumbbell domains (IID)) decrease successively, and their Hip(2)(T) are the same.