We investigated the effects of molecular weight and film thickness on the crystallization and microphase separation in semicrystalline block copolymer polystyrene-

block

-poly(L-lactic acid) (PS-

b

-PLLA) thin films

at the early stage of film evolution (when

T

g

<

T

<

T

ODT

) by

in situ

hot stage atomic force microscopy. For PS-

b

-PLLA 1 copolymer which had lower molecular weight and higher PLLA fraction

diffusion-controlled break-out crystallization started easily. For PS-

b

-PLLA 2 with higher molecular weight

crystallization in nanometer scales occurs in local area. After melting of the two copolymer films

islands were observed at the film surface: PS-

b

-PLLA 1 film was in a disordered phase mixed state while PS-

b

-PLLA 2 film formed phase-separated lamellar structure paralleling to the substrate. Crystallization-melting and van der Waals forces drove the island formation in PS-

b

-PLLA 1 film. Film thickness affected the crystallization rate. Crystals grew very slowly in much thinner film of PS-

b

-PLLA 1 and remained almost unchanged at long time annealing. The incompatibility between PS and PLLA blocks drove the film fluctuation which subsequently evolved into spinodal-like morphology.