Crystal patterns in ultrathin films of six poly(ethylene oxide) fractions with molecular weights from 25000 to 932000 g/mol were characterized within crystallization temperature range from 20℃ to 60℃. Labyrinthine

dendritic and faceted crystal patterns were observed in different temperature ranges

and then labyrinthine-to-dendritic and dendritic-to-faceted transition temperatures TL-D and TD-F were quantitatively identified. Their molecular weight dependences are

T

L-D

(

M

w

) =

T

L-D

(∞) −

K

L-D

/

M

w

where

T

L-D

(∞) = 38.2℃ and

K

L-D

= 253000℃·g/mol and

T

D-F

(

M

w

) =

T

D-F

(∞) −

K

D-F

/

M

w

where

T

D-F

(∞) = 54.7℃ and

K

D-F

= 27000℃·g/mol. Quasi two-dimensional blob models were proposed to provide empirical explanations of the molecular weight dependences. The labyrinthine-to-dendritic transition is attributed to a molecular diffusion process change from a local-diffusion to diffusion-limited-aggregation (DLA) and a polymer chain with

M

w

≈ 253000 g/mol within a blob can join crystals independently. The dendritic-to-faceted transition is attributed to a turnover of the pattern formation mechanism from DLA to crystallization control

and a polymer chain with a

M

w

≈ 27000 g/mol as an independent blob crosses to a depletion zone to join crystals. These molecular weight dependences reveal a macromolecular effect on the crystal pattern formation and selection of crystalline polymers.