use rollingStyle and datePattern.
<appender name="FileAppender" type="log4net.Appender.RollingFileAppender">
<file type="log4net.Util.PatternString" value="C:\log4net\Log.log" />
<appendToFile value="true" />
<rollingStyle value="Date" />
<datePattern value="yyyyMM" />
<layout type="log4net.Layout.PatternLayout">
<conversionpattern value="%level %date{dd MMM yyyy HH:mm:ss,fff} %logger - %message%newline" />
</layout>
</appender>