Social tensions have marred the parliament vote, which was called after the scandal-tainted Berlusconi's party quit the ruling left-right coalition and he was ejected from parliament over a tax fraud conviction.
Truckers have staged protests across Italy this week in an anti-austerity movement led by the "Forconi" (Pitchforks) group, with activists joining in sometimes violent demonstrations in several cities.
The government won some breathing space from this week's news that the economy has stopped contracting and an end to the country's longest post-war recession is imminent but social misery is widespread.
Leaders of the protest have promised to stage a "peaceful siege until the politicians go away" if the government wins the confidence vote as expected.
Transport Minister Maurizio Lupi said the vote would give a mandate for the government, which was only formed in April after a two-month stalemate, "to do concrete things with even more courage".
The uneasy coalition government was saved from certain collapse following Berlusconi's exit last month after a group of the playboy politician's former proteges broke from him and chose to stay in the cabinet.
The vote also comes after the election of the 38-year-old mayor of Florence Renzi to the leadership of the Democratic Party, the main centre-left party.
Renzi has promised to work together with Letta, who is also from the Democratic Party, although he has said he will pressure for more cuts in Italy's overweight bureaucracy and more help for crisis-hit Italians.
According to Italian media, Letta is aiming for a "German-style pact" that will include a list of measures to be adopted in the year to come.
He was expected to outline this programme to fight unemployment and revive economic growth in an address to parliament later on Wednesday.
Another major hurdle will be an overhaul of Italy's electoral law, which was widely blamed for the inconclusive result of a general election in February.
Italy's constitutional court earlier this month declared parts of the law unconstitutional, fuelling calls for an immediate change that could give the country some much-needed political stability.
The voting on Wednesday will begin in the Chamber of Deputies lower house, where Letta enjoys a comfortable majority, and continue in the Senate upper house, where he may only win by around a dozen votes.