Krabi is the closest international airport to Koh Lanta, and can be reached by direct flights from Bangkok, Singapore or Kuala Lumpur as well as Koh Samui.  Trang has a domestic airport and is about the same distance from Koh Lanta.  If you arrive by car or bus, you must cross from the mainland by a short scenic ferry to Koh Lanta Nai and then another, even shorter ferry to Koh Lanta Yai.  The trip including ferry crossings, takes about 2 hours from either Trang or Krabi Airports and about 4.5 hours from Phuket International airport.  Minibus and taxi transportation are readily available from all 3 airports.  Another option is to come by the daily ferry from Phuket which takes about 3.5 hours and arrives directly at Koh Lanta Yai.