Security will be a main issue in both countries, particularly in the Philippines, Asia's only majority Catholic country, where up to six million people are expected to attend an outdoor Mass on Jan. 18.
Up to 40,000 police, troops and reservists will take part in what military chief General Gregorio Catapang has called the country's biggest ever security operation.
"There will be soldiers rappelling up and down helicopters to rescue the pope in case he will be pinned down by a sea of people. We may airlift or use naval boats to bring the pope to safety if necessary," he said.
When Pope John Paul visited Manila in 1995, security perimeters were breached and he had to be taken by helicopter to a Mass site because his car could not get through a sea of some 5 million people.
One theme of the Jan. 12-19 trip will be climate change. During his stay in the Philippines he will visit Tacloban, where Typhoon Haiyan killed 6,300 people in 2013.
Sri Lanka is among the Asian countries experts say will see sea level rises likely to displace people and adversely affect tourism and fisheries.
The Vatican says Francis, who is preparing an encyclical on the environment, will speak about the issue several times.
While Pope John Paul made a number of trips to Asia - visiting both countries in 1995 - Francis' immediate predecessor Benedict, who resigned in 2013, made none to a region the Vatican sees as a potential growth area.
"We have to recover the presence of a pope in this preponderant area of humanity," Vatican spokesman Father Federico Lombardi said. Only about 3 percent of people in the region are Catholic.
"This continent in many ways represents a frontier for the Church," said Father Antonio Spadaro, editor of the Italian Jesuit magazine Civilta Cattolica. "Inter-religious dialogue is tested every day and young Churches there are growing".
The 78-year-old arrives on Tuesday morning in the Sri Lankan capital, Colombo, days after President Mahinda Rajapaksa lost his bid for a third term, ending a decade of rule that critics say had become authoritarian and marred by nepotism and corruption.
Lombardi said he hoped the surprise election result in the former British colony would not give rise to any "inconveniences that will affect the serenity and tranquillity of the trip".
The main purpose of the three-day stop in Sri Lanka is to canonise Joseph Vaz, a Catholic priest credited with rebuilding the Church there in the 17th and 18th centuries after Dutch occupiers imposed Calvinism as the official religion.
The Indian Ocean island nation is about 70 percent Buddhist, 13 percent Hindu, 10 percent Muslim and only about 7 percent Catholic. Francis will stress the need for worldwide inter-religious dialogue, and, speaking after the recent attacks in France, again condemn the concept of violence in God's name.
He will also preach a message of reconciliation during a visit to Madhu, in the north that was the centre of a 26-year civil war that ended with the defeat of ethnic Tamil rebels in 2009.
Vatican officials say that despite its minority status, the Church in Sri Lanka can help reconciliation because it includes members of both ethnic groups - Sinhalese and Tamil.
Francis arrives on Thursday in the Philippines, where more than 80 percent of people are Catholic.
One main topic in the former Spanish colony will be the effect of immigration on the family. The search for jobs outside the country - mostly in domestic work - has put strains on many families.