不要自己做Arff文件，麻烦，你做成.csv，让weka自己另存为就行了。

文本文件打开，把字段名复制到第一段，保存成.csv

举个例子：
@RELATION cars

@ATTRIBUTE buying REAL
@ATTRIBUTE maint REAL
@ATTRIBUTE doors REAL
@ATTRIBUTE persons REAL
@ATTRIBUTE lug-boot REAL
@ATTRIBUTE safety REAL
@ATTRIBUTE class {unacc,acc,good,v-good}

@DATA
vhigh,vhigh,2,2,small,low,unacc
vhigh,vhigh,2,2,small,med,unacc
vhigh,vhigh,2,2,small,high,unacc
vhigh,vhigh,2,2,med,low,unacc
vhigh,vhigh,2,2,med,med,unacc
vhigh,vhigh,2,2,med,high,unacc
vhigh,vhigh,2,2,big,low,unacc
vhigh,vhigh,2,2,big,med,unacc

然后用Weka打开就可以了。