Traveling from Montreal to Quebec City by bus is fairly easy. It takes about 3 hours, and there are departures every hour on the hour. Once in Quebec City, you can choose to stop in Sainte-Foy, which is where the university, malls and some hotels are, or downtown Quebec, which is closer to the old city area. A round-trip for adults costs about $75. The main advantage of traveling by bus compared with traveling by train is that it's much cheaper (about half-price) and there are more departures everyday. You don't have to make a reservation either, they'll bring in an extra bus if they have too so no worries!
Big old bus station, but it’s real clean, has outdoor seating, nice places to do some shopping, buying food, etc inside. Nice water fountain out front with more seating. Just a bus station, but a pleasant place to be—all bus stations I’ve been to are usually pretty grungy, not this. Nice welcome to the city.
Video monitors inside will tell you which location your bus will be leaving from. All express buses to Montreal leave on the half hour, and make a stop at the other bus location on the outskirts of Quebec. This bus station in Quebec to Montreal takes 3 hours.
The most efficient public transport from Montreal to Quebec is the ORLÉANS EXPRESS bus service. As you may have guessed by its name, it is an express service from Montreal to Quebec (and vice versa). Buses depart every hour on the hour from Montreal and every hour at 30 minutes past the hour from Quebec. The ride takes around 3 hours (not much longer than by car), maybe less if there is no traffic.
The terminal in Montreal is the Voyageur bus terminal, located at the intersection of Berri St. and De Maisonneuve Blvd (it is also directly accessible from the Berri-UQAM metro station). The terminal in Quebec, which has a much more refined name (Gare des Palais), is located across the park from Saint-Paul St. just outside the city fortifications in Old Quebec.
Please keep in mind there are stops in Longueuil (10 minutes from Montreal) and Sainte-Foy (15 minutes from Quebec) along the way, so make sure you get off at the right place! Also, make sure you arrive a half-hour before departure time if you have no reservation.
If going directly to Quebec from the Montreal Pierre-Elliott-Trudeau airport, there are also 4 bus departures from the airport to Quebec. Check out the website below for more information.
I used the Orleans Express bus line to come to Quebec City from Montreal.... they have a very good and comfortable service, it takes 3 hours to get to the Gare Du Palais from Montreal and there are buses departing every hour back and forth.
Do arrive at the bus station with good anticipation prior to your departure cause my bus back from Quebec to Montreal departed 2-3 minutes before the scheduled time! It also gets pretty crowded (during rush hours) so the earlier you arrive the better the seat you can choose.
Reaching Québec city is pretty easy, as it works as a communication node from the Southern Québec province to the Northern parts. For an instance, there's a bus every hour from Montréal. The bus station is located just at the bottom of the old city hill, thus making a visit to the walls and the most historical places here an easy election.