The　effect　of　average　feedstock　powder　size　on　the　evolution　of　microstructure　and　thermal　conductivity　are　comprehensively　analyzed.　It　was　found　that　the　porosity　and　thermal　conductivity　of　the　LaYbZr2O7　coatings　were　positively　correlated　with　the　average　powder　size.　The　conventional　notion　that　higher　porosity　results　in　lower　thermal　conductivity　is　challenged　by　the　above　results.　The　abnormal　porosity　and　thermal　conductivity　relationship　can　be　ascribed　to　the　density　of　horizontal　cracks　which　dominates　the　thermal　conductivity　reduction.　Reducing　the　average　powder　size　leads　to　a　greater　concentration　of　horizontal　cracks　and　hence　reduced　thermal　conductivity.　Moreover,　the　LaYbZr2O7　coatings　not　only　show　significantly　lower　initial　thermal　conductivity　of　under　0.70　W　m-1　K-1,　but　also　exhibit　stronger　sintering　resistance　in　terms　of　porosity　stability　and　thermal　insulation　ability　compared　to　the　conventional　YSZ　coatings.　The　present　work　affords　a　new　insight　in　developing　advanced　thermal　barrier　coatings　with　ultralow　thermal　conductivity.