The country’s first full congress in decades is being closely watched for any shift in policies or changes in political leadership.
Mr Kim is expected to reassert his nuclear ambitions, amid speculation he will soon conduct a fifth nuclear test.
Foreign media have been invited but are not allowed inside the venue.
The capital was spruced up ahead of the event and citizens layed flowers in central squares as it got under way.
The streets are lined with National and Workers’ Party flags with banners that read “Great comrades Kim Il-sung and Kim Jong-il will always be with us” and “Defend the headquarters of the Korean revolution at the cost of the our lives”.
It is the seventh meeting of North Korea’s Worker’s Party and the first since 1980, and is being held inside the April 25 House of Culture, now covered in vast red and gold banners and massive images of the current leader’s father and grandfather.
This year’s event is shrouded in secrecy. The BBC’s Stephen Evans is one of about 100 foreign journalists invited but says reporters are being closely monitored.
Kim Jong-un is inside the hall, our correspondent adds, with guards lined up outside.
But instead of being allowed into the congress, reporters have instead been taken to on a factory tour.
The agenda and duration of the event is not known but experts say Kim Jong-un is likely to declare his so-called “byongjin” policy, which is the simultaneous push towards economic development and nuclear capability.
It could also see a new generation of leaders put in place.
The meeting will elect a new central committee, which appoints a Politburo – the central decision-making body of the Communist party – and many say loyalists to the current leader will be rewarded with high profile posts.