The　study　on　the　size　variation　of　coal　particles　has　significance　for　the　reaction　results　and　process　simulations　of　coal　pyrolysis　and　gasification.　The　influencing　factors,　such　as　the　moisture,　volatile,　pyrolysis　time,　medium　atmosphere　and　heat　stress　effect,　on　the　size　variation　(0.61-6.0　mm)　of　low　volatile　Fujian　coal　particles,　were　investigated　in　a　fixed　bed　under　the　pyrolysis　temperature　900°C.　The　results　indicate　that　the　heat　stress,　resulting　from　the　temperature　difference　between　coal　inner　and　its　outer　surface,　is　a　dominate　influencing　factor　on　the　size　variation　of　coal　particles　in　the　pyrolysis　process.　However,　the　evaporation　of　moisture　and　volatilization　of　volatile　in　coal　exhibit　little　influence.　The　size　variation　of　coal　particles　can　be　completed　in　just　1　minute　during　pyrolysis　and　about　5%-20%　coal　particles　will　break　into　smaller　particles　in　pyrolysis　process.　For　the　original　coal　with　larger　size　(＞2.5　mm),　the　coal　size　in　pyrolysis　reduces　obviously　when　the　moisture　of　coal　reaches　up　to　15%,　but　for　the　original　coal　with　smaller　size　(＜　1.6　mm)　the　coal　size　in　pyrolysis　does　not　change　at　different　moistures.　It　also　shows　that　the　medium　atmosphere　in　pyrolysis　has　a　stronger　effect　on　the　size　variation　of　coal　particles.　The　stream　in　coal　gasification　can　cause　more　fragments　in　the　coal　particles　and　reduce　their　size　more　than　that　of　the　air　in　combustion.