Magnetic transitions and magnetoresistance effect of the HfFe6Ge6-type Y0.9La0.1Mn6Sn6 compound have been investigated in the temperature range of 5-380K. The sample displays antiferromagnetic behaviour in the whole temperacan be induced by an applied field. The metamagnetic transition field decreases monotonically from 2T at 5K to 0.4T at 300K. The giant magnetoresistance effect is observed with the metamagnetic behaviour, such as -10.4% at 245K under a field of 5T.