The Calgary-based company's load factor slipped 0.7 point from a year ago to 78.5 per cent in May, indicating slightly less of its total seating capacity was used.
However, WestJet's capacity rose by 9.1 per cent or 161 million available seat miles and traffic was up 8.2 per cent or 114 million revenue passenger miles.
The airline says that translates into about 110,000 additional guests compared with May 2012.
WestJet has been forming and strengthening partnerships with foreign carriers as it prepares to launch its Encore regional service on June 24.
Its main rival, Montreal-based Air Canada (TSX:AC.B), launched a regional turboprop service called Air Canada Express in February and is also planning a new secondary airline flying jets to be called Rouge.