Bolted flanged connections have been widely used in a lot of plants such as chemical, petrochemical, nuclear and so on. Sometimes, leakage accidents occur from the bolted flanged connections in the plants. Thus, it is important to tighten a lot of bolts and nuts precisely in bolted flanged connections. However, it is difficult to tighten the bolts and nuts at target bolt preloads because a big scatter of bolt preloads occur due to the difference in the friction coefficients at the bearing surfaces and the contact surfaces in the engaged screw threads. Thus, better tightening workers are needed for better tightening bolts and nuts in a lot of connections with gaskets. In Japan, better workers for tightening bolts and nuts decrease because population in Japan decreases. Some better tightening workers are in bigger companies and it is difficult to keep them in smaller companies. In Japan, some companies of big plants want to keep the better tightening workers, thus they want to establish a certification system for keeping the better tightening workers. However, no certification system has been established. In our sealing technology committee, a working group started to investigate the certification system 3 years ago. Firstly, knowledge on the sealing technology which tightening workers should learn has been developed, that is, mechanical characteristics of bolted flanged connections and leakage phenomena. In addition, a method how to calculate a target bolt preload, bolt tightening torque is described in the text. Furthermore, two tightening procedures, that is, one way tightening specified in Japanese standard (JIS B 2251) and star sequence method specified in ASME PCC-1 will be studied. Actually, workers excise to tighten bolts and nuts in connections according to the above two ways. The workers also learn how to attach strain gauges at the bolts and the flange hub. Of course, they learn measuring methods of the stresses and the strains. They also learn a measuring method of an amount of leakage using 4 inch pipe flange. Now, the certification system is discussed in WG committee, in particular, problems are who can certificate the tightening workers, how to determine training facilities and the detail of training items. A new trial for establishing the certification system and the training items will be achieved as fast as we can for better tightening workers and the manager for bolted flange connections.