Recently it has become a common technique for the more economical production of acetate tow to use a finer d.p.f. (denier per filament) and lower total denier tow together with a sacrificed filter firmness. This study was undertaken to define the effects of fiber weight, plasticizer content and RCR (Residual Crimp Ratio) on cold and hot hardness of filters. Hot hardness here is defined as values which subtract the % hot collapse from 100, which was measured using a Rhodia HDS-4 hardness tester. Also, the collapsing behavior is investigated and compared with the conventional smoking pattern. By the fiber weight reduction, hot hardness decreases to a large extent though cold hardness also decreases also slightly. It is found that the pattern of hot collapse is similar to the pattern of moisture condensation on filter tips. Therefore, we conclude that moisture in smoke is the active agent which causes hot collapse of acetate filter. Increasing the plasticizer content normally improves cold hardness, but it has an adverse effect on hot hardness improvement. To improve both cold and hot hardness at the target pressure drop, it is effective to make filters at Higher RCR. However, it is known that Higher RCR increases pressure drop variation because of the deterioration uniformity of fiber configuration. A new plug making technology of using a modified tow transport jet results in a higher RCR with an uniform fiber configuration and gives an extremely improved hot hardness.