We present the results of a 6.4 square degrees imaging survey of the Pleiades cluster in the I and Z -bands .
The survey extends up to 3 degrees from the cluster center and is 90 \% complete down to I \simeq 22 .
It covers a mass range from 0.03 M _ { \odot } to 0.48 M _ { \odot } and yields 40 brown dwarf candidates ( BDCs ) of which 29 are new .
The spatial distribution of BDCs is fitted by a King profile in order to estimate the cluster substellar core radius .
The Pleiades mass function is then derived accross the stellar-substellar boundary and we find that , between 0.03 M _ { \odot } and 0.48 M _ { \odot } , it is well represented by a single power-law , dN / dM \propto M ^ { - \alpha } , with an index \alpha = 0.60 \pm 0.11 .
Over a larger mass domain , however , from 0.03 M _ { \odot } to 10 M _ { \odot } , the mass function is better fitted by a log-normal function .
We estimate that brown dwarfs represent about 25 % of the cluster population which nevertheless makes up less than 1.5 % of the cluster mass .
The early dynamical evolution of the cluster appears to have had little effect on its present mass distribution at an age of 120 Myr .
Comparison between the Pleiades mass function and the Galactic field mass function suggests that apparent differences may be mostly due to unresolved binary systems .