Context : Supernova remnants ( SNRs ) were often discovered in radio surveys of the Galactic plane .
Because of the surface-brightness limit of previous surveys , more faint or confused SNRs await discovery .
The Sino-German \lambda 6 cm Galactic plane survey is a sensitive survey with the potential to detect new low surface-brightness SNRs .

Aims : We want to identify new SNRs from the \lambda 6 cm survey map of the Galactic plane .

Methods : We searched for new shell-like objects in the \lambda 6 cm survey maps , and studied their radio emission , polarization , and spectra using the \lambda 6 cm maps together with the \lambda 11 cm and \lambda 21 cm Effelsberg observations .
Extended polarized objects with non-thermal spectra were identified as SNRs .

Results : We have discovered two new , large , faint SNRs , G178.2 - 4.2 and G25.1 - 2.3 , both of which show shell structure .
G178.2 - 4.2 has a size of 72 \arcmin \times 62 \arcmin with strongly polarized emission being detected along its northern shell .
The spectrum of G178.2 - 4.2 is non-thermal , with an integrated spectral index of \alpha = -0.48 \pm 0.13 .
Its surface brightness is \Sigma _ { 1 ~ { } GHz } = 7.2 \times 10 ^ { -23 } { Wm ^ { -2 } Hz ^ { -1 } sr ^ { -1 } } , which makes G178.2 - 4.2 the second faintest known Galactic SNR .
G25.1 - 2.3 is revealed by its strong southern shell which has a size of 80 \arcmin \times 30 \arcmin .
It has a non-thermal radio spectrum with a spectral index of \alpha = -0.49 \pm 0.13 .

Conclusions : Two new large shell-type SNRs have been detected at \lambda 6 cm in an area of 2200 deg ^ { 2 } along the the Galactic plane .
This demonstrates that more large and faint SNRs exist , but are very difficult to detect .