North Korea has a cool continental climate, with usually warm humid summers and long harsh winters. When packing for North Korea you should definitely pay attention to what season you are going in as there is a large variation.
Summer
Summer days are typically around 25 degrees, however can get up to over 30 degrees and can be very humid. So if you travel in summer, shorts and t-shirts are advised, or other cool clothing items. Nights at this time are very pleasant.
Spring and autumn
It is best to pack some warm clothes, as nights at this time can still be cool, and depending on how close you are to winter it can still be quite cold.
Winter
Winter can be very cold in North Korea with temperatures sometimes reaching below -20 degrees Celcius. You should pack thermal underwear, gloves and hats to keep yourself warm during these times. If possible waterproof boots are also advised as snow is common.
Formal Wear
There will be several occasions where you will be required to wear formal clothing in North Korea, such as when you see the statues of the leaders, and when paying respects at the Palace of the Sun. This means long pants for men and dresses for women (women can also wear long pants too) and clothing with no writing or rips.
Footwear
Depending on the itinerary of your trip, you may need walking shoes, but in general, you'll want comfortable shoes for walking around the city. For colder months we recommend winter boots, and for the formal situations simple smart shoes (it would be best to wear some sort of dark coloured shoes and not runners).
Luggage
If you are travelling by train it is a good idea to bring a smaller amount of luggage as there may be limited space. A single large suitcase is fine but you may have to lift it up to put it in luggage storage areas on the train. If you are going in and out of Beijing, you can leave your large luggage in storage in Beijing and take a smaller bag for your trip to North Korea. Luggage storage is available at Beijing airport and possibly your place of accommodation if you are staying in Beijing before/after the tour.
Medicines
It is a good idea to bring all medications along with their prescriptions (if required) to North Korea as they may not be available at North Korean pharmacies. Most medications in North Korea will either be local, or imported from China and/or Russia.